thickness has 1 distinct synonym spread across 1 of its 4 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 thinness.
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.
indistinct articulation
“judging from the thickness of his speech he had been drinking heavily”
Synonyms, antonyms and the broader and narrower relations for thickness come from Princeton University’s WordNet. Full sources and licences.
resistance to flow