the organ inside the body where food goes when you swallow it; the front part of the body below the chest stomach pains an upset stomach (British English also) a stomach upset exercises to strengthen the stomach muscles It's not a good idea to drink (= alcohol) on an empty stomach (= without having eaten anything). You shouldn't exercise on a full stomach. The attacker kicked him in the stomach. Lie on your stomach with your arms by your side. They crawled along the ground on their stomachs.