smoothness has 5 distinct synonyms spread across 2 of its 5 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 roughness.
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”
the quality of being bland and gracious or ingratiating in manner
the quality of having a level and even surface
“the water was a glassy smoothness”
the quality of being free from errors or interruptions
“the five-speed manual gearbox is smoothness personified”
Synonyms, antonyms and the broader and narrower relations for smoothness come from Princeton University’s WordNet. Full sources and licences.