• 胃痛いつう — stomachache
• 胃炎いえん — gastritis
• 胃酸いさん — gastric acid
• 胃腸いちょう — stomach and intestines
• 胃壁いへき — stomach lining

The character 胃 consists of the ⺼ (flesh) radical and a simplified pictograph indicating a container or organ. It clearly represents an internal digestive organ, and is frequent in anatomical and medical terms.

胃いが 痛いたいので 病院びょういんに 行いきました。
I had stomach pain, so I went to the hospital.

彼かれは 胃腸いちょうが 弱よわいです。
He has a weak stomach and intestines.