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.
Synonyms, antonyms and the broader and narrower relations for smoothness come from Princeton University’s WordNet. Full sources and licences.
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”