variation has 11 distinct synonyms spread across 6 of its 11 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 instance of change; the rate or magnitude of change
the angle (at a particular location) between magnetic north and true north
the process of varying or being varied
(astronomy) any perturbation of the mean motion or orbit of a planet or satellite (especially a perturbation of the earth's moon)
the act of changing or altering something slightly but noticeably from the norm or standard
“who is responsible for these variations in taxation?”
Synonyms, antonyms and the broader and narrower relations for variation come from Princeton University’s WordNet. Full sources and licences.