tension has 5 distinct synonyms spread across 3 of its 6 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.
(psychology) a state of mental or emotional strain or suspense
“he suffered from fatigue and emotional tension”
the physical condition of being stretched or strained
“it places great tension on the leg muscles”
Synonyms, antonyms and the broader and narrower relations for tension come from Princeton University’s WordNet. Full sources and licences.
a balance between and interplay of opposing elements or tendencies (especially in art or literature)
“there is a tension created between narrative time and movie time”
(physics) a stress that produces an elongation of an elastic physical body
“the direction of maximum tension moves asymptotically toward the direction of the shear”
feelings of hostility that are not manifest
“he could sense her latent hostility to him”
the action of stretching something tight
“tension holds the belt in the pulleys”