Entry 1 sense Webster, 1913 Daydream /(-drēm`)/ · Day·dream · IPA /ˈdeɪdɹiːm/ 01 n. A vain fancy speculation; a reverie; a castle in the air; unfounded hope. 1. A vain fancy speculation; a reverie; a castle in the air; unfounded hope. “Mrs. Lambert's little daydream was over.” — Thackeray.