1. Gas in the digestive, or intestinal tract: Flatus from the lower intestinal tract contains hydrogen, methane, skatoles (white crystalline organic compound having a strong fecal odor), indoles (bacterial decomposition in the intestine), carbon dioxide, and small amounts of oxygen and nitrogen.
2. The expelling of gas, or air, from a body orifice, especially the anus: The passing of flatuses may average a dozen a day in some people and up to a hundred times in others.

The foul smell usually is caused by small traces of gases, such as hydrogen sulfide, ammonia, and methane.