Flatulence is the expulsion through the rectum of a mixture of gases that are byproducts of the digestion process of mammals and other animals. The medical term for the mixture of gases is flatus, informally known as a fart, or simply (in American English) gas. The gases are expelled from the rectum in a process colloquially referred to as "passing gas", "breaking wind" or "farting". Flatus is brought to the rectum by the same peristaltic process which causes feces to descend from the large intestine. The noises commonly associated with flatulence are caused by the vibration of the anal sphincter, and occasionally by the closed buttocks.

Nitrogen, the main constituent of air, is the primary gas released during flatulence, along with carbon dioxide. The lesser component gases methane and hydrogen are flammable, and so flatus containing adequate amounts of these can be ignited. However, not all humans produce flatus that contains methane. For example, in one study of the feces of nine adults, only five of the samples contained archaea capable of producing methane. Similar results are found in samples of gas obtained from within the rectum.