cheerfulness has 4 distinct synonyms spread across 2 of its 2 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 2 recorded opposites are uncheerfulness, cheerlessness.
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 being cheerful and dispelling gloom
“flowers added a note of cheerfulness to the drab room”
Synonyms, antonyms and the broader and narrower relations for cheerfulness come from Princeton University’s WordNet. Full sources and licences.
a feeling of spontaneous good spirits
“his cheerfulness made everyone feel better”