Vegetarianism is a dietary practice excluding all body parts of any animal and products derived from animals (e.g., lard, tallow, gelatin, cochineal) from one's diet. Many contemporary vegetarian diets include some honey as well as milk and other dairy products, and some include eggs.
Different practices of vegetarianism include:
Religious dietary restrictions come in many forms and are sometimes compatible with the secular terminology; see below.
The following are not generally considered vegetarianism:
In current English, the term "vegetarian" is occasionally used for restricted diets that nevertheless include some types of meat. Usually these deviations from traditional usage are made casually, perhaps for lack of a better word. The resulting confusion of terms can create awkward situations for more strict vegetarians, however, as any traditional vegetarian who has been expected to eat a dish because it "only contains a little meat" (or "is just fish") can readily attest.
In 1847, attendees at the meeting of the first Vegetarian Society in Ramsgate, England, agreed that a "vegetarian" was a person who refuses to consume flesh of any kind. Prior to that time, vegetarians had often been called Pythagoreans, after the philosopher and his followers who also abstained from meat (and possibly some types of beans).
Vegetarian societies (apart from India) were first formed in majority meat eating European countries both as a means to promote the diet and to gather together vegetarians for mutual support. By 2000, most western and developing nations had functioning vegetarian societies. The countries that were first to establish societies are still the ones most likely to have the greatest proportion of vegetarians within their populations.
The first societies were:
The International Vegetarian Union  , a union of all the national societies was founded in 1908.
A majority of the world's vegetarians follow the practice for religious reasons. Many religions, including Buddhism, Hinduism, Taoism and especially Jainism, teach that ideally life should always be valued and not willfully destroyed for unnecessary human gratification.
Jews, Christians and Muslims are all left with the biblical ideal of the "Garden of Eden" diet, which from all appearances is strictly vegan (cf. Gen. 1:29, 9:2-4; Is. 11:6-9). However, only minorities within these populations actually practice and advocate such strict diets, since the same book of the Bible, Genesis, later gives permission to Noah's descendants to consume animal flesh, but not without great suffering simultaneously administered to all creatures: "The fear and dread of you will fall upon all the beasts of the earth and all the birds of the air, upon every creature that moves along the ground, and upon all the fish of the sea" (Gen. 9:2). Suffice to say, the Judeo-Christian God's permission for humankind to eat meat was not an unmixed or otherwise "unqualified" blessing. It was a concession, with penalties--not the least of which was, most probably, a dramatically decreased life expectancy (see Gen. 6:3). (Noah's great-grandfather, Methuselah, is famously reported as having lived an amazing 969 years, prior to the dawn of God-authorized human meat-eating.)
In the Bible, the Book of Genesis teaches that human beings were originally vegetarian, but that later, following the Deluge, God permitted people to eat meat as well. Many Judeo-Christian vegetarians interpret this to mean that God originally intended human beings to be vegetarians, and that people would do well to be vegetarians, even though meat-eating is permitted. Additionally, some Biblical prophecy suggests that in the Messianic age , there will be universal vegetarianism, even among normally carnivorous animals. (For example, Isaiah 11:7 says, "The cow will feed with the bear, their young will lie down together, and the lion will eat straw like the ox.")
Rabbinical Judaism discourages ascetic practices in general. With respect to food, this teaching may be summarized by the Talmudic statement, "Man will have to account for everything he saw but did not eat." To Jewish vegetarians wishing to remain consistent with this teaching, vegetarianism is not a form of self-deprivation, because the vegetarian does not desire to eat meat and believes it is healthier not to eat meat. On the other hand, the Talmud discourages indulgence and states that it is preferable that one's diet consist mostly of non-meat products. There are several arguments from Judaism used by Jewish vegetarians. One is that, since Adam and Eve were not allowed to eat meat and that, according to some opinions, in the Messianic era, the whole world will be vegetarian, not eating meat is something that brings the world closer to that ideal. A second one is that the laws of shechita are meant to prevent the suffering of animals and today, with factory farming, even kosher slaughterhouses are considered by some authorities not to fulfil enough of the requirements to render the meat kosher. A third one is that the Sages only mandated eating an olive's bulk of meat during festivals, but even then, this was because in Talmudic times, meat was considered essential for one's diet (whereas a vegetarian will probably be of the opinion that current science has shown otherwise).
In Christianity, Paul wrote in his Epistle to the Romans that although he himself ate meat, the choice to eat meat or abstain from meat should be a matter of personal conviction: "The man who eats everything must not look down on him who does not, and the man who does not eat everything must not condemn the man who does, for God has accepted him." (Romans 14:3) Several Christian monastic groups have encouraged vegetarianism, including the Desert Fathers , Trappists, Benedictines, and Carthusians. Some Protestant groups, such as Seventh-day Adventists, take a literal interpretation of the Biblical prophecies of universal vegetarianism and encourage vegetarianism as a preferred though not required lifestyle. (However, most evangelical groups are unaware of the existence any such prophecies, and point instead to the explicit prophecies of temple sacrifices in the Messianic Kingdom, many of which are eaten--see Ezekiel 46:12 where peace offerings and freewill offerings will be offered, and Leviticus 7:15-20 where it states that such offerings are eaten). In the nineteenth century, members of the Bible Christian sect established the first vegetarian groups in England and the United States.
Islam explicitly permits the eating of some kinds of meat. The hadith collection of al-Nasa'i recounts an episode wherein several of Mohammed's companions wish to practice various ascetic practices including Censored page, vegetarianism, and extreme fasting, and Mohammed rebukes them all. Since in Islam, it is forbidden to forbid that which is permitted, some Islamic scholars conclude that vegetarianism is forbidden. Muslim vegetarians and their supporters, however, make a distinction between choosing not to do something and forbidding it, and argue that a Muslim may choose to be a vegetarian, but as an aesthetic or ethical consideration and not as a religious duty. However, islam see little difference between religious duty and ethical duty and therefore, this view is not widely accepted.
Rastafarians generally follow a diet called "I-tal", which eschews the eating of food that has been artificially preserved, flavoured, or chemically altered in any way. Many Rastafarians consider it to also forbid the eating of meat.
Hindus of certain castes (especially Brahmins) are forbidden from consuming anything gained at the expense of an animal's suffering: e.g. meat, eggs, animal byproducts such as rennet and gelatin (including gelatin capsules) and honey. The milk of cows, buffalo and goats as well as dairy products (other than cheese containing rennet) are acceptable, as milk is given willingly. Leather from cows who have died of natural causes is acceptable. (Note: The diet of the orthodox Hindu also excludes alcohol, as well as "overly-stimulating" foods such as onions and garlic.) However, not all Hindus are vegetarian.
In Chinese societies, "simple eating" (素食 su4shi2) refers to a particular restricted diet associated with Taoist monks, and sometimes practiced by members of the general population during Taoist festivals. It is referred to by the English word "vegetarian"; however, though it rejects meat, eggs and milk, this diet does include oysters and oyster products.
Many Westerners think that Buddhist precept against killing implies that Buddhist should avoid eating the meat of animals. However, this is to miss the distinction between killing of animal and eating of already dead meat. And during the Buddha's time, there was no general rule requiring monks to refrain from eating meat. In Pali scriptures there are several recorded instance of Buddha eating meat, though whether Buddha died from eating tainted pork is disputed. In fact, at one point the Buddha specifically refused to make such a rule, declaring that one can eat meat as long as one does not hear, see or suspect that meat is specifically killed for oneself. And these rules were invoked in relation to commercial purchase of meat in an episode involving General Shia. Buddha also stated that it is one's immoral intents that makes one impure, not the eating of meat, and declared meat eating as karmically neutral.
However the situation is very different in the case of Mahayana Buddhism. Though Mahayana Buddhism accepts Theravadan sutras as valid, in their own Mahayana sutras, the account of Buddha eating meat is absent. Secondly, at the time when Mahayana Buddhists were formulating their monastical rules, monks and nuns no longer received their food by begging. Instead, they lived in a monastery, where food was sent to them from outside by the lay community. So, if meat was offered, it was specifically killed and prepared for monks, which violates Buddha's rule. Thirdly, Mahayana Buddhism places great emphasis on the Boddhisattva way, where the cultivation of compassion is the central focus of the practice. In Mahaparinirvana , it is stated that "the eating of meat extinguishes the seed of great compassion". In addition, a passage in the Lankavatara Sutra shows the Buddha inveighing strongly in favor of vegetarianism, though the historical accuracy of this passage is strongly disputed. Therefore, meat eating came to be greatly discouraged in early Mahayana schools. This is still the case in Chinese Buddhism, while many Japanese and Korean schools has adopted different interpretations of this issue. In Vajrayana Buddhism, tantric practice is said to purify one regardless of one's diet.
A belief that continues in Theravada Buddhism from its Vedic roots is that the killing of larger animals results in more bad karma than that of smaller ones, e.g. that it would be less bad to kill a chicken than a cow. This is due to the greater intention and effort required to kill the larger animal. Furthermore, fish are considered of lesser importance than mammals.
In the modern Buddhist world, attitudes toward vegetarianism vary by location. In China and Vietnam, monks typically eat no meat. In Japan or Korea some schools do not eat meat, while most do. Theravadan Sri Lanka and Southeast Asia do not practice vegetarianism. All Buddhists however, including monks, are allowed to practice vegetarianism if they wish to do so.
In the Bahá'í Faith a vegetarian diet, although not required, is often considered preferable. Furthermore, Bahá'ís believe "Fruits and grains" will be the foods of the future and the time will come when meat will no longer be eaten .
Many people believe that the production of meat and animal products at current and likely future levels is environmentally unsustainable. It is also argued that even if sustainable, modern industrial agriculture is changing ecosystems faster than they can adapt. While vegetarian agriculture produces some of the same problems as animal production, the environmental impact of animal production is significantly greater. 
Free-range animal production requires land for grazing, which has prompted encroachment on undeveloped lands and clear cutting. The move into wild lands has increased the rate of species extinction and damaged the services offered by nature, such as natural processing of pollutants. Overgrazed lands lose their ability to support animal production, which makes further agricultural expansion necessary. Factory farm animal production, while having a smaller land-use footprint, requires large quantities of feed that must be grown over large areas of land. Both free-range and concentrated animal production require large quantities of fresh water and energy, which are currently taken from nonrenewable sources such as aquifers and fossil fuels. Animal production also creates damaging animal waste. In the United States (the world's largest emitter of greenhouse gases) livestock account for nearly 20% of total methane emissions. 
Compare this with economic vegetarians, who consider the meat industry economically unsound.
"Rain forests are still being felled to graze hamburger cattle. Going vegan saves one acre of forest every year."- Cornell University
"On irrigated land, 1lb of vegetables uses 25 gallons. 1lb of beef uses 5,214 gallons." - University Of California
"The world's 17 major fisheries are on the point of environmental collapse because of over-fishing" - United_Nations
"If Britain went vegetarian, less than half the farm land would be needed - vegan, less than a quarter" - Reading University
Citing the same efficiency concerns as environmentalist vegetarians and economic vegetarians, many vegetarians see natural resources as being freed up by vegetarianism, particularly veganism.
A popular saying is that even with more food, the problem is shipping all of that food to the starving people. Yet, petroleum is one of the resources freed up for other usage by a vegan diet: Within the Pulitzer-winning book by John Robbins, "Diet for a New America," which uses data primarily sourced from the world's largest body of scientists, AAAS , Robbins explains how the petroleum used in the transportation of farm-animals, the later processing of them, and the raising and harvesting of the vast amount of crops fed to farm-animals (which is much greater than the amount of crops people would need if we were to eat the crops directly, rather than feeding them to animals, then eating the animals), adds up to greatly increase the amount of petroleum used. So, if more people adopt a vegan diet, not only is more food available, but more petroleum to deliver that food is.
"35 per cent of the world's people can be fed on a meat-based diet. A plant diet could feed everyone - then plus some." - Sir Crispin Tickell
"It takes about 10kg of good quality plant protein - such as wheat and soy - to produce 1kg of meat protein."
"The amount of veg protein fed to the US beef herd would feed almost the entire populations of India and China - two billion people."
"Veg consumption is much more efficient than growing feed for meat production and dairying." - WHO
"Amount of grain needed to end extreme hunger - 40 million tonnes. Amount of grain fed to animals in the West - 540 million tonnes." - United_Nations
"90 per cent of the UK's animal feed protein concentrates come from poor countries - often those where children die from starvation."
Some vegetarians believe that the production and consumption of meat and animal products are inappropriate treatments of animals. Reasons for believing this are varied, and may include a belief in animal rights or an aversion to inflicting harm on other living things. Some people believe the treatment which animals receive in the production of meat and animal products justifies never eating meat or animal products, even if alternatives do not exist. Others believe that if alternative means of survival exist, it is ethical to choose an alternative, such as vegetarianism. With the exception of a small minority of people, e.g., nomadic hunting and herding societies such as the Inuit and Sami, everyone is free to choose not to eat meat or animal products.
According to reputable sources such as the American Dietetic Association, American Heart Association, British Medical Association, and the Mayo Clinic, vegetarian diets offer a number of health benefits compared to non-vegetarian diets. Vegetarians as a group compared to non-vegetarians have lower body mass indices, lower levels of cholesterol, lower blood pressure, and less incidence of heart disease, hypertension, type 2 diabetes, some forms of cancer, renal disease, dementia, and osteoporosis.  
As for weight loss, in a year-long study comparing Dean Ornish's vegetarian diet to Weight Watchers, The Zone Diet , and The Atkins Diet, Dean Ornish's diet showed the most weight-loss. (source: Dansinger, M.L., Gleason, J. L., Griffith, J.L., et al., "One Year Effectiveness of the Atkins, Ornish, Weight Watchers, and Zone Diets in Decreasing Body Weight and Heart Disease Risk", Presented at the American Heart Association Scientific Sessions November 12, 2003 in Orlando, Florida.)
"Vegetarians have lower rates of obesity, coronary heart disease, high blood pressure, large bowel disorders, cancers and gallstones." - British_Medical_Association
"Vegetarians often live longer and suffer less from several chronic diseases." - ADA
"Vegetarians have half to three-quarters the risk of dying of heart disease compared to non-vegetarians." - PCRM
"Cancer rates among vegetarians are 25-50 per cent less than non-vegetarians." - PCRM
"Hypertension (high blood pressure) in vegetarians is one third to one half that of meat eaters." - PCRM
"Diabetes is much less likely to be a cause of death in vegetarians" - ADA
"Vegetarians have a much lower incidence of caesarean section." - PCRM
"95 per cent of all food poisoning comes from meat and animal products" - British_Medical_Association
For more health information, including peer-reviewed citations regarding specific nutrients, see the separate entry for vegetarian nutrition.
Some vegetarians are vegetarian because they were raised by vegetarians. Others may have become vegetarians because of a vegetarian partner, family member, or friend. Some people live in regions that are predominantly vegetarian (such as Gujarat), making meat-eaters a minority. When removed from the social influences that cause vegetarianism, some people will stop being vegetarian while others will remain vegetarian.
Social aspects also influence some vegetarians who believe slaughterhouses have a negative psychological and socializing effect on the employees charged with slaughtering the animals.
While vegetarianism is commonly defined strictly on the basis of dietary intake, many religiously, ethically or environmentally motivated vegetarians, in common with the animal rights and Green movements, try to minimise the harm done to animals in all aspects of their lives.
Many religiously motivated vegetarians consider the avoidance of skin contact with products made from body parts (e.g., leather, tallow soap) an integral part of their definition of vegetarianism. Others consider leather made from the skin of animals who died of natural causes acceptable. While for many Hindus it is impractical, there are those who shy away completely from the use of leather articles made from cowhide. Some state and cities in India have even banned cow-slaughter in places of pilgrimage or whole regions based on the sentiments of some Hindus.
Resources for vegetarians:
Resources for vegetarian history: