relaxation has 10 distinct synonyms spread across 6 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.
(physiology) the gradual lengthening of inactive muscle or muscle fibers
(physics) the exponential return of a system to equilibrium after a disturbance
a feeling of refreshing tranquility and an absence of tension or worry
“the easiness we feel when sleeping”
an occurrence of control or strength weakening
“the relaxation of requirements”
freedom from activity (work or strain or responsibility)
“took his repose by the swimming pool”
a method of solving simultaneous equations by guessing a solution and then reducing the errors that result by successive approximations until all the errors are less than some specified amount
Synonyms, antonyms and the broader and narrower relations for relaxation come from Princeton University’s WordNet. Full sources and licences.