whim

(noun) a sudden desire. Synonyms: caprice, impulse. Example: "He bought it on an impulse."

(noun) an odd or fanciful or capricious idea. Synonyms: notion, whimsy, whimsey. Examples: "The theatrical notion of disguise is associated with disaster in his stories." "He had a whimsy about flying to the moon." "Whimsy can be humorous to someone with time to enjoy it."