ionisation has 1 distinct synonym spread across 2 of its 2 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 condition of being dissociated into ions (as by heat or radiation or chemical reaction or electrical discharge)
“the ionization of a gas”
the process of ionizing; the formation of ions by separating atoms or molecules or radicals or by adding or subtracting electrons from atoms by strong electric fields in a gas
Synonyms, antonyms and the broader and narrower relations for ionisation come from Princeton University’s WordNet. Full sources and licences.