smooth has 16 distinct synonyms spread across 7 of its 12 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 rough, roughen.
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.
having a surface free from roughness or bumps or ridges or irregularities
“smooth skin”
of the margin of a leaf shape; not broken up into teeth
of motion that runs or flows or proceeds without jolts or turbulence
“a smooth ride”
free from obstructions
“smooth the way towards peace negotiations”
the act of smoothing
“he gave his hair a quick smooth”
1 of the 12 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for smooth.
Synonyms, antonyms and the broader and narrower relations for smooth come from Princeton University’s WordNet. Full sources and licences.