"secret" en el diccionario Collins English Cobuild & Thesaurus

secret
adj

1 backstairs, behind someone's back, camouflaged, cloak-and-dagger, close, closet (informal) concealed, conspiratorial, covered, covert, disguised, furtive, hidden, hole-and-corner (informal) hush-hush (informal) reticent, shrouded, undercover, underground, under wraps, undisclosed, unknown, unpublished, unrevealed, unseen

2 abstruse, arcane, cabbalistic, clandestine, classified, cryptic, esoteric, mysterious, occult, recondite

3 hidden, out-of-the-way, private, retired, secluded, unfrequented, unknown

4 close, deep, discreet, reticent, secretive, sly, stealthy, underhand
n

5 code, confidence, enigma, formula, key, mystery, recipe, skeleton in the cupboard

6 in secret  behind closed doors, by stealth, huggermugger  (archaic) in camera, incognito, secretly, slyly, surreptitiously

Antonyms
,   adj apparent, candid, disclosed, exoteric, frank, manifest, obvious, open, overt, public, straightforward, unconcealed, visible, well-known