According to the Columbia Encyclopedia, the acid in a person's stomach is hydrochloric acid. It is one of the components of gastric juice, the strongly acidic and nearly colorless liquid produced by glands in the stomach lining.
The essential components of gastric juice also include mucus and the digestive enzymes known as pepsin and rennin. The Columbia Encyclopedia states that the acidity of hydrochloric acid helps maintain the environment most conducive to the effective activity of pepsin, which in turn plays an important role in the digestive process, converting proteins into substances that are more easily absorbed. Mucus works to protect the stomach lining from the action of gastric juice. Rennin helps to digest milk proteins.