zero has 17 distinct synonyms spread across 5 of its 10 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 point on a scale from which positive or negative numerical quantities can be measured
the sight setting that will cause a projectile to hit the center of the target with no wind blowing
adjust (an instrument or device) to zero value
3 of the 10 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for zero.
Synonyms, antonyms and the broader and narrower relations for zero come from Princeton University’s WordNet. Full sources and licences.