secret

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Kept hidden from knowledge or view; concealed.
• adjective Not expressed; inward.
• adjective Given to keeping one's thoughts and activities unknown to others; secretive.
• adjective Not revealing a secret or not given to revealing secrets.
• adjective Operating in a hidden or confidential manner.
• adjective Containing information, the unauthorized disclosure of which poses a grave threat to national security.
• adjective Not much visited; secluded.
• adjective Known or shared only by the initiated.
• adjective Beyond ordinary understanding; mysterious.
• noun Something that is kept out of the knowledge or sight of others or is known only to oneself or a few.
• noun Something that remains beyond understanding or explanation; a mystery.
• noun A method or formula for doing or making something well, especially when not widely known.
• noun A variable prayer said after the Offertory and before the Preface in the Mass.
• idiom (in secret) Without others knowing.

from The Century Dictionary.

• Set or kept apart; hidden; concealed.
• Privy; not decent to be exposed to view.
• Occult; mysterious; not seen; not apparent: as, the secret operations of physical causes.
• Affording privacy; retired; secluded; private.
• Close, cautious, or discreet in speech, or as regards the disclosure of one's own or another's affairs; faithful in keeping secrets; not given to blabbing or the betrayal of confidence; secretive; reticent.
• Synonyms and Secret, Latent, Private, Covert, Occult, Clandestine, hidden, concealed, covered, shrouded, veiled, obscure, recondite, close, unknown. The last four of the italicized words, and in their primary sense the participles, express intentional concealment; the others do not. Secret is the most general, but expresses complete concealment. Latent, literally lying concealed, may mean hidden from those most concerned: as, I had a latent sense, feeling, or desire; hence its appropriateness in the expression latent heat. Private (as, it was kept strictly private) emphasizes the fact that some know the thing in question, while others are kept in ignorance. Covert— that is, covered—suggests something underhand or well put out of sight: as, a covert motive, sneer, irony: it is opposed to frank or avowed. Occult suggests mystery that cannot be penetrated: as, the occult operations of nature; occult arts. Clandestine is now always used for studious or artful concealment of an objectionable or dishonorable sort: as, a clandestine correspondence: it applies especially to action.
• noun Something studiously hidden or concealed; a thing kept from general knowledge; what is not or should not be revealed.
• noun A hidden, unrevealed, unexplained, or unex-plainable thing; a mystery.
• noun The key or principle by the application of which some difficulty is solved, or that which is not obvious is explained or made clear; hidden reason or explanation.
• noun Secrecy.
• noun In liturgics, a variable prayer in the Roman and some other Latin liturgies, said secretly (see secretly) by the celebrant after the offertory, etc., and immediately before the preface.
• noun plural The parts of the body which propriety requires to be concealed.
• noun A concealed piece or suit of armor. Persons fearing assassination sometimes wear such defenses beneath their ordinary dress.
• noun A skull-cap of steel worn sometimes under and sometimes over the camail.
• noun A skeleton cap of slender steel bars, affording a good defense against a blow, worn within a hat or other head-covering.
• noun A secret device or contrivance.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Hidden; concealed.
• adjective Withdrawn from general intercourse or notice; in retirement or secrecy; secluded.
• adjective rare Faithful to a secret; not inclined to divulge or betray confidence; secretive.
• adjective obsolete Separate; distinct.
• noun Something studiously concealed; a thing kept from general knowledge; what is not revealed, or not to be revealed.
• noun A thing not discovered; what is unknown or unexplained; a mystery.
• noun The parts which modesty and propriety require to be concealed; the genital organs.
• noun in a private place; in privacy or secrecy; in a state or place not seen; privately.
• transitive verb obsolete To keep secret.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective not open or public; kept private or not revealed
• adjective conducted with or marked by hidden aims or methods
• noun something that baffles understanding and cannot be explained
• adjective not expressed
• adjective having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding
• adjective designed to elude detection
• noun something that should remain hidden from others (especially information that is not to be passed on)