Triviality is something unsignificant and of small
importance
View page
He thought being on time was a triviality.
Don't waste time on trivialities.
View page
triviality
View page
Seth. has written:
'Truth and triviality' -- subject(s): English poetry
View page
For weight meaning heaviness, the opposite could be lightness.
If it is used metaphorically to mean importance, the opposite is
unimportance or triviality.
In engineering, the opposite of a weight (load) can be a
counterweight.