daydream; waking dream
A set phrase meaning a dream or fantasy experienced while awake, often with a nuance of something unrealistic or fleeting.
彼は授業中、真昼の夢を見ていた。
He was daydreaming during class.
Compound of 真昼 (midday) and 夢 (dream), literally 'midday dream', referring to a dreamlike state while awake.