fixation has 5 distinct synonyms spread across 3 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 abnormal state in which development has stopped prematurely
an unhealthy and compulsive preoccupation with something or someone
Synonyms, antonyms and the broader and narrower relations for fixation come from Princeton University’s WordNet. Full sources and licences.
the activity of fastening something firmly in position
(histology) the preservation and hardening of a tissue sample to retain as nearly as possible the same relations they had in the living body