secret has 23 distinct synonyms spread across 9 of its 14 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.
conducted with or marked by hidden aims or methods
“clandestine intelligence operations”
designed to elude detection
“a hidden room or place of concealment such as a priest hole”
(of information) given in confidence or in secret
“this arrangement must be kept confidential”
having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding
“mysterious symbols”
something that should remain hidden from others (especially information that is not to be passed on)
“the combination to the safe was a secret”
information known only to a special group
“the secret of Cajun cooking”
4 of the 14 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for secret.
Synonyms, antonyms and the broader and narrower relations for secret come from Princeton University’s WordNet. Full sources and licences.
something that baffles understanding and cannot be explained
“how it got out is a mystery”