seriousness vs trait

seriousness and trait both are nouns.

Word	Noun	Adjective	Verb	Adverb
seriousness	Yes	No	No	No
trait	Yes	No	No	No

As nouns, trait is a hypernym of seriousness; that is, trait is a word with a broader meaning than seriousness:

seriousness (noun)	trait (noun)
the trait of being serious	a distinguishing feature of your personal nature
the quality of arousing fear or distress
an earnest and sincere feeling