calm has 23 distinct synonyms spread across 7 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 2 recorded opposites are agitate, stormy.
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.
make calm or still
“quiet the dragons of worry and fear”
become quiet or calm, especially after a state of agitation
“After the fight both men need to cool off.”
cause to be calm or quiet as by administering a sedative to
“The patient must be sedated before the operation”
steadiness of mind under stress
“he accepted their problems with composure and she with equanimity”
(of weather) free from storm or wind
“calm seas”
Synonyms, antonyms and the broader and narrower relations for calm come from Princeton University’s WordNet. Full sources and licences.