(n.) A vain fancy speculation; a reverie; a castle in the air; unfounded hope.

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