solidness has 5 distinct synonyms spread across 4 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
the state in which a substance has no tendency to flow under moderate stress; resists forces (such as compression) that tend to deform it; and retains a definite size and shape
Synonyms, antonyms and the broader and narrower relations for solidness come from Princeton University’s WordNet. Full sources and licences.
the quality of being substantial or having substance
the quality of being solid and reliable financially or factually or morally
“the solidity of the evidence worked in his favor”