secret

[1]	arcanum
[2]	private/confidential matter/affair , mystery , enigma , mystery , closed book
[3]	in secret , privately , confidentially , secretly , on the q.t.or Q.T , surreptitiously , under cover , by stealth , stealthily , furtively , quietly , on the sly , clandestinely

adj.

[1]	concealed , hidden , private , covert , shrouded , clandestine , confidential , covert , quiet , under cover , secretive , unpublishable , unpublished , (Colloq) hush-hush
[2]	cryptic , private , arcane , mysterious , incomprehensible , esoteric , recondite , abstruse , cryptographic , encrypted , encoded
[3]	clandestine , cloak-and-dagger , hole-and-corner , hugger-mugger , hush-hush , surreptitious , undercover , underground
[4]	unavowed
[5]	private
[6]	hidden
[7]	privy , secluded
[8]	confidential
[9]	mysterious , mystic , mystical , occult , orphic