Several parts combine into one character.

胃 is a semantic compound: 田 on top (originally a pictograph of a sack of grain — not actually "field" here) + 月 (flesh radical) below. The composite literally pictures: "a grain-sack inside the body = stomach." A rare case where Chinese etymology drew an organ exactly by its function — the stomach as the body's grain-holding pouch, sketched in seal script as a small bag dangling above flesh.

Korean reading "wi." 胃腸 (wi-jang, stomach and intestines / digestive system), 胃酸 (wi-san, stomach acid), 胃癌 (wi-am, stomach cancer), 健胃 (geon-wi, "to strengthen the stomach" — a category in traditional Korean medicine and on Korean digestive-aid product labels), 胃炎 (wi-yeom, gastritis). Korean medical Korean uses 胃 universally.

Mandarin wèi, 4th tone. 胃 (wèi), 胃口 (wèikǒu, "stomach-mouth" = appetite — daily idiom), 胃疼 (wèiténg, stomach ache), 胃癌 (wèi'ái, stomach cancer). The compound 胃口 deserves attention: it is the Chinese way to say "I have an appetite for that" or "that whets my appetite." Mandarin reads body-state through the stomach rather than the brain — 我对这个没胃口 (wǒ duì zhège méi wèikǒu) = "I have no appetite for this" = "I'm not interested in this," a metaphorical reach you don't hear in Korean or Japanese.

Japanese on-reading イ (i) — no kun reading in common use. 胃 (i, stomach — the bare character is itself a daily word in Japanese), 胃腸 (ichō, stomach and intestines), 胃癌 (igan, stomach cancer), 胃酸 (isan, stomach acid). The single-character reading 胃 (i) appears as the prominent banner in Japanese pharmaceutical advertising for any stomach medicine — TV commercials with the giant red character い are a Japanese cultural staple that anyone who has watched 30 seconds of Japanese television will recognize.

Memory aid: a grain-sack (田) above flesh (月) — the body's pouch for what we eat.