The stomach is part of the digestive system. It is an internal organ between the esophagus and the small intestines. The stomach is the third stage in the digestive process. It holds food after ingestion. Food in the stomach then passes through to the small intestine where most of the food's nutrition is absorbed.
The stomach contains hydrochloric acid made by stomach cells. As a useful side-effect, the acid kills bacteria in the food. However, its main function is to help the protein-digesting enzymes called proteases work. In general, the food is churned, squeezed and mixed. This prepares it for digestion. The partially digested food is called chyme.
The stomach acid may cause problems. It can make peptic ulcers worse. It can also cause heartburn—pain in the chest when acid from the stomach refluxes (backs up into) the oesophagus. These can usually be treated by neutralizing the acid. Vomiting occurs when food goes back up the oesophagus and out the mouth. Vomiting can be a symptom of disease.
Stomachs of other mammals work differently to human stomachs. Ruminants, for example, have a stomach with multiple compartments. In that system, food goes through the first part of the digestive system twice, and the work of digesting the grass is done by bacteria.
Other words for stomach include belly and tummy.