Daydream(n.) A vain fancy speculation; a reverie; a castle in the air; unfounded hope.
Thesaurus Entries
Walter Mitty, absence of mind, absentmindedness, absorption, abstractedness, abstraction, airy nothing, autism, bad dream, be absent, bemusement, brown study, bubble, castle-building, chimera, conceiving, daydreamer, daydreaming, deception, deluded belief, delusion, depth of thought, dereism, divagate, dream, dream of, dream vision, dreaming, dreamland, dreamworld, engrossment, false belief, fantasy, fantasying, fit of abstraction, freak out, go woolgathering, ignis fatuus, illusion, imagination, imagining, incubus, misbelief, misconception, moon, mooning, moonraking, muse, musefulness, musing, muted ecstasy, nightmare, nostalgia, phantasm, pipe dream, pipe-dream, pipe-dreaming, preoccupation, reverie, self-deceit, self-deception, self-delusion, stargaze, stargazing, stray, study, trance, trick, trip, vapor, vision, wander, wishful thinking, wishfulness, wistful eye, wistfulness, woolgathering, wrong impression, yearnfulness