Levity

[Tina]

The quality of weighing less than something else of equal
bulk; relative lightness, especially as shown by rising through, or
floating upon, a contiguous substance; buoyancy; -- opposed to gravity.

[Raeanne Dimick]

Inappropriate humor during matters of seriousness to lessen the dread of reality

[Zipporah Schumaker]

The opposite of gravity.

[Renato]

The treatment of a serious matter with humour or lack of due respect.

[Justin Bieber]

humour or lack of seriousness, especially during a serious occasion: