approximation has 4 distinct synonyms spread across 2 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.
an approximate calculation of quantity or degree or worth
“an estimate of what it would cost”
the quality of coming near to identity (especially close in quantity)
an imprecise or incomplete account
“newspapers gave only an approximation of the actual events”
the act of bringing near or bringing together especially the cut edges of tissue
Synonyms, antonyms and the broader and narrower relations for approximation come from Princeton University’s WordNet. Full sources and licences.