1. Kept hidden from knowledge or view; concealed.
2. Dependably discreet.
3. Operating in a hidden or confidential manner: a secret agent.
4. Not expressed; inward: their secret thoughts.
5. Not frequented; secluded: wandered about the secret byways of Paris.
6. Known or shared only by the initiated: secret rites.
7. Beyond ordinary understanding; mysterious.
8. Containing information, the unauthorized disclosure of which poses a grave threat to national security.

1. Something kept hidden from others or known only to oneself or to a few.
2. Something that remains beyond understanding or explanation; a mystery.
3. A method or formula on which success is based: The secret of this dish is in the sauce.
4. Secret A variable prayer said after the Offertory and before the Preface in the Mass.

Synonyms: These adjectives mean deliberately hidden from view or knowledge. Secret is the most general: a desk with a secret compartment; secret negotiations.
Stealthy suggests quiet, cautious deceptiveness intended to escape notice: heard stealthy footsteps on the stairs.
Covert describes something that is concealed or disguised: protested covert actions undertaken by the CIA.
Clandestine implies stealth and secrecy for the concealment of an often illegal or improper purpose: clandestine intelligence operations.
Furtive suggests the slyness, shiftiness, and evasiveness of a thief: a menacing and furtive look to his eye.
Something surreptitious is stealthy, furtive, and often unseemly or unethical: the surreptitious mobilization of troops preparing for a sneak attack.
Underhand implies unfairness, deceit, or slyness as well as secrecy: achieved success by underhand methods.