levity

Short definition from Wiktionary

Noun

1. 1
   A lightness of manner or speech, frivolity; flippancy; a lack of appropriate seriousness; an inclination to make a joke of serious matters.
2. 2
   A lack of steadiness.
3. 3
   The state or quality of being light, buoyancy.
4. 4
   A lighthearted or frivolous act.