seriousness has 4 distinct synonyms spread across 3 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is frivolity.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the quality of arousing fear or distress
“he learned the seriousness of his illness”
the trait of being serious
“a lack of solemnity is not necessarily a lack of seriousness”
Synonyms, antonyms and the broader and narrower relations for seriousness come from Princeton University’s WordNet. Full sources and licences.