The word that means a fanciful notion or impulse is whimor whimsy. An antonym for whimsy would be seriousness or deliberation.

The base is whim, which forms a similar noun whimsy, and hence whimsical.

whim

The word "whimsical" originates from the late 17th century from the noun "whim-wham," meaning a strange or fanciful object or thing. Over time, "whimsical" came to describe a playful or quirky behavior or idea.

Synonyms of the word notion: 1. a general or vague idea. 2. an opinion or belief. 3. a whim. Pls. double-check by looking it up in your dictionary. ^_^