noun (n)

• an odd or fanciful or capricious idea(noun.cognition)

  Example:
  He had a whimsy about flying to the moon.
  Whimsy can be humorous to someone with time to enjoy it.
  

  Synonym:
  notion, whim, whimsey
  

  source: wordnet30
• the trait of acting unpredictably and more from whim or caprice than from reason or judgment(noun.attribute)

  Synonym:
  arbitrariness, capriciousness, flightiness, whimsey, whimsicality
  

  source: wordnet30
• A whimsey.(noun)

  source: webster1913