tense has 2 distinct synonyms spread across 3 of its 8 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. Its 3 recorded opposites are relax, relaxed, lax.
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.
increase the tension on
“alternately relax and tense your calf muscle”
become tense, nervous, or uneasy
“He tensed up when he saw his opponent enter the room”
in or of a state of physical or nervous tension
pronounced with relatively tense tongue muscles (e.g., the vowel sound in `beat')
taut or rigid; stretched tight
“tense piano strings”
a grammatical category of verbs used to express distinctions of time
Synonyms, antonyms and the broader and narrower relations for tense come from Princeton University’s WordNet. Full sources and licences.