verb
(transitive) To form a mental image of something; to envision or create something in one's mind.
(transitive) To believe in something created by one's own mind, often something false.
(transitive) To assume; to suppose.
(transitive) To conjecture; to guess.
(intransitive) To use one's imagination.
(transitive, obsolete) To contrive in purpose; to scheme; to devise.
(transitive, Internet slang, rhetorical, sarcastic) Used to mock an idea by suggesting that it is ridiculous or ill thought through.
noun
(fandom slang) A short fanfic or prompt placing a reader insert in a novel scenario with a character or celebrity.