Several parts combine into one character.

In oracle bone script, 夢 shows a person lying on a bed with a hazy shape — something like exaggerated eyebrows or unclear visual figures — drifting around the head. The picture is precisely a sleeper experiencing something they cannot quite see clearly. Ancient Chinese understood the dream as the visual blur on the closed eye, and the character preserves that vagueness in its very strokes. Japan and Korea retain 夢; Mainland simplified to 梦.

The most famous philosophical use of 夢 is 莊周夢蝶 — Zhuangzi's "butterfly dream" — where the philosopher cannot tell whether he is a man who dreamed of being a butterfly, or a butterfly now dreaming of being a man. The Korean compound 胡蝶夢 (hojeopmong) preserves this entire 4th-century-BCE allegory in three characters.

Korean reading "mong." 夢想 (mongsang, daydream / fantasy), 惡夢 (angmong, nightmare), 吉夢 (gilmong, "auspicious dream" — a culturally important concept; Korean grandmothers traditionally interpret pregnancy dreams as 태몽), 胡蝶夢 (hojeopmong, Zhuangzi's butterfly dream).

Mandarin mèng, 4th tone (simplified 梦). 做梦 (zuòmèng, "to make a dream" = to dream), 梦想 (mèngxiǎng, dream / aspiration — used in motivational and political contexts), 噩梦 (èmèng, nightmare), 美梦 (měimèng, sweet dream). The phrase 中国梦 (zhōngguó mèng, "the China Dream") became a major political slogan in modern Chinese rhetoric — paralleling the American "American Dream."

Japanese on-reading ム (mu) — 夢想 (musō, daydream), 夢中 (muchū, "in the middle of a dream" = absorbed / engrossed in something — extremely common idiom). The kun-reading ゆめ (yume) is the everyday word: 夢を見る (yume o miru, "to see a dream" = to dream), 正夢 (masayume, a dream that comes true), 初夢 (hatsuyume, the first dream of the New Year — auspicious in Japanese tradition).

Memory aid: a sleeper with hazy figures around the head — the very shape of dreaming.