Metabolism, in the most general sense, is the ingestion and breakdown of complex compounds, coupled with the liberation of energy, and the consequent generation of waste products. It is a major process of living organisms, and because this process can happen at many levels within an organism, we can identify several kinds of metabolism:

• When concerning an organism in its entirety, metabolism (also called total metabolism) is all of the living organism's chemical processes. The organism's metabolism can be dichotomized into the synthesis of organic molecules (anabolism) and their breakdown (catabolism).
• When concerning a particular substance, metabolism (also called specific metabolism) is the chemical activity involving this substance in a living organism. This is commonly the digestion of food, and the disposal of wastes.
• When concerning a particular living cell, metabolism (also called cell metabolism) is all of the chemical processes in that cell.

The word metabolism comes from μεταβολισμος, the Greek word for "change". An abstract definition of metabolism is difficult to make in the same way an abstract definition of life is difficult to make. For example, according to the definition above, fire has a metabolism, too (it "eats", for example, wood, converts it to heat, and disposes ashes), but we would not describe fire as metabolizing.

Santorio Santorio weighed himself in a chair suspended from a steelyard balance, before and after a meal, and published the results in Ars de statica medecina, 1614, the first controlled experiment in human metabolism (illustration, upper left).