whimsey

(noun) an odd or fanciful or capricious idea. Synonyms: notion, whim, whimsy. 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."

(noun) the trait of acting unpredictably and more from whim or caprice than from reason or judgment. Synonyms: flightiness, arbitrariness, whimsicality, whimsy, capriciousness. Example: "I despair at the flightiness and whimsicality of my memory."