fitness has 2 distinct synonyms spread across 2 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 unfitness.
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 suitable
“they had to prove their fitness for the position”
Synonyms, antonyms and the broader and narrower relations for fitness come from Princeton University’s WordNet. Full sources and licences.
good physical condition; being in shape or in condition