fantasy

n. product of the imagination, fancy; illusion

v. imagine, simulate, daydream, portray in the mind

Fantasy is a genre that uses magic and other supernatural forms as a primary element of plot, theme, and/or setting. The genre is generally distinguished from science fiction and horror by overall look, feel, and theme of the individual work, though there is a great deal of overlap between the three (collectively known as speculative fiction). In its broadest sense, fantasy comprises works by many writers, artists, filmmakers, and musicians, from ancient myths and legends to many recent works embraced by a wide audience today.
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Noun
1. imagination unrestricted by reality; "a schoolgirl fantasy"
(synonym) phantasy
(hypernym) imagination, imaginativeness, vision
(hyponym) pipe dream, dream
(derivation) fantasize, fantasise
2. fiction with a large amount of fantasy in it; "she made a lot of money writing romantic fantasies"
(synonym) phantasy
(hypernym) fiction
(hyponym) science fiction
3. something many people believe that is false; "they have the illusion that I am very wealthy"
(synonym) illusion, phantasy, fancy
(hypernym) misconception
(hyponym) bubble
(derivation) fantasize, fantasise

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A castle in the air.
Meaning something is impossible to carry out. Said of illusions and visionary prospects.