fluency has 4 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 disfluency.
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.
powerful and effective language
“his eloquence attracted a large congregation”
skillfulness in speaking or writing
the quality of being facile in speech and writing
Synonyms, antonyms and the broader and narrower relations for fluency come from Princeton University’s WordNet. Full sources and licences.