In etymology, the process of back-formation is the creation of a neologism by reinterpreting an earlier word as a compound and removing the spuriously supposed affixes. The resulting new word is also called a back-formation.
For example, the suffix holic used in words like workaholic, chocoholic, Censored page arises through back-formation by reinterpreting alcoholic as alco plus holic, although in reality the word is derived from alcohol. Likewise Watergate was simply the name of a hotel, but "gate" was reinterpreted as a suffix and later used in naming other scandals.
Many words came into English by this route: Pease was once a mass noun but was reinterpreted as a plural, leading to the back-formation pea; the noun statistic was likewise a back-formation from the field of study statistics, and asset was a back-formation from assets (originally Anglo-Norman asetz). In Britain the word burgle came into use in the 19th century as a back-formation from burglar (in America burglarize is used).
Sometimes such changes occur in more than one stage. The word utopia, from the Greek for "nowhere", was reanalyzed as eu-topia, "good place"; replacing eu with dys as is commonly done, the word dystopia ("bad place", an ideally horrible world) was coined.
Even though many legitimate English words are formed this way, new coinages are often frowned upon, and are often used for humorous effect. For example, gruntled or pervious (from disgruntled and impervious) would be considered mistakes today, and used only in humorous contexts. (The comedian George Gobel regularly used original back-formations in his humorous monologues.) But burger (and beefburger, cheeseburger, etc., from hamburger) is in common use today though it would have been considered awkward or colloquial as late as the 1940s; and enthuse (from enthusiasm) is gaining popularity, though it is still considered substandard by some today--it will likely be in common use within a few years, particularly in its non-transitive form, "He enthused over the presents."
Homo sapiens is Latin for thinking man and is in fact singular according to strict grammar rules (plural would be homines sapientes,) but many incorrectly take homo sapiens to be plural, and homo sapien to be singular.
See also: backronym