The stomach is a muscular, hollow, dilated part of the digestion system which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects (mid-gut), and molluscs. It is involved in the second phase of digestion, following mastication (chewing). The stomach is located between the esophagus and the small intestine.

Stomach is particularly interesting in association with drinking coffee for various reasons. Coffee is a stimulant of the gastric acid. Some of the compounds found in coffee such as caffeine, catechols, and N-alkanoly-5-hydroxytryptamides, up-regulate the gastric secretion. Another compound in coffee, N-methylpyridinium (N-MP), was shown to down-regulate the secretion.

Because of this property, coffee has been touted as a digestion aid after heavy meals. On the other hand, this causes stomach problems for some coffee drinkers, because the stomach will secrete too much acid.