消化系统（消化道）气体或胃肠气，俗称屁，指动物消化道肛门排出的气体。有研究指出，牛羊等反刍动物放屁所产生的甲烷可能导致全球暖化。

屁的主要气体来源有二：

1. 咀嚼时吞入消化道的空气；
2. 大肠和直肠内的细菌，特别是大肠杆菌，发酵食物残渣而产生的气体。

Flatulence is defined in the medical literature as "flatus expelled through the anus" or the "quality or state of being flatulent", which is defined in turn as "marked by or affected with gases generated in the intestine or stomach; likely to cause digestive flatulence". The root of these words is from the Latin flatus – "a blowing, a breaking wind". Flatus is also the medical word for gas generated in the stomach or bowels. These standard definitions do not reflect the fact that a proportion of intestinal gas may be composed of swallowed environmental air, and hence flatus is not totally generated in the stomach or bowels. The scientific study of this area of medicine is termed flatology.