fusion has 7 distinct synonyms spread across 5 of its 7 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 of being combined into one body
the merging of adjacent sounds or syllables or words
a nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy
the combining of images from the two eyes to form a single visual percept
correction of an unstable part of the spine by joining two or more vertebrae; usually done surgically but sometimes done by traction or immobilization
the act of fusing (or melting) together
Synonyms, antonyms and the broader and narrower relations for fusion come from Princeton University’s WordNet. Full sources and licences.