se·cret 
(sēkrĭt)
(sēkrĭt)adj.
1.
a. Kept hidden from knowledge or view; concealed: a secret identity; a secret passageway.
b. Not expressed; inward: secret desires.
2.
a. Given to keeping one's thoughts and activities unknown to others; secretive: "Scrooge ... was secret, and self-contained, and solitary as an oyster" (Charles Dickens).
b. Not revealing a secret or not given to revealing secrets: "She boasted ... that he did tell her. But he didn't. He was secret as the grave" (Ruth Prawer Jhabvala).
3.
a. Operating in a hidden or confidential manner: a secret commission; a secret agent.
b. Containing information, the unauthorized disclosure of which poses a grave threat to national security.
4. Not much visited; secluded: a secret hiding place.
5.
a. Known or shared only by the initiated: secret rites.
b. Beyond ordinary understanding; mysterious: "like Pan, calling out with his flute to come join in on the secret chaos of the world" (Rick Bass).
n.
Idiom: 1. Something that is kept out of the knowledge or sight of others or is known only to oneself or a few: wanted to have no secrets between them.
2. Something that remains beyond understanding or explanation; a mystery: unlocking the secrets of the atom.
3. A method or formula for doing or making something well, especially when not widely known: 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.
in secret
Without others knowing.
[Middle English, from Old French, from Latin sēcrētus, from past participle of sēcernere, to set aside : sē-, apart; see s(w)e- in the Appendix of Indo-European roots + cernere, to separate; see krei- in the Appendix of Indo-European roots.]
secret·ly adv.
Synonyms: secret, stealthy, covert, clandestine, furtive, surreptitious, underhanded
These adjectives mean deliberately hidden from view or knowledge. Secret is the most general: The desk has a secret compartment. The spies conduct secret negotiations.
Stealthy suggests quiet, cautious deceptiveness intended to escape notice: "The males of each community formed temporary parties that regularly patrolled their borders and sometimes made stealthy incursions into the territory of their neighbors" (Jane Goodall).
Covert describes something that is concealed or disguised: Students protested the covert actions undertaken by the CIA.
Clandestine implies stealth and secrecy for the concealment of an often illegal or improper purpose: "The gold was becoming so routine that he'd stopped being clandestine about it" (Jennifer Egan).
Furtive suggests the slyness, shiftiness, and evasiveness of a thief: I took a furtive glance at the papers on the desk.
Something surreptitious is stealthy, furtive, and often unseemly or unethical: "She takes surreptitious sips from a flask in her bag as she waits in the cold" (Mary V. Dearborn).
Underhand implies unfairness, deceit, or slyness as well as secrecy: The politician achieved success by underhand methods.
These adjectives mean deliberately hidden from view or knowledge. Secret is the most general: The desk has a secret compartment. The spies conduct secret negotiations.
Stealthy suggests quiet, cautious deceptiveness intended to escape notice: "The males of each community formed temporary parties that regularly patrolled their borders and sometimes made stealthy incursions into the territory of their neighbors" (Jane Goodall).
Covert describes something that is concealed or disguised: Students protested the covert actions undertaken by the CIA.
Clandestine implies stealth and secrecy for the concealment of an often illegal or improper purpose: "The gold was becoming so routine that he'd stopped being clandestine about it" (Jennifer Egan).
Furtive suggests the slyness, shiftiness, and evasiveness of a thief: I took a furtive glance at the papers on the desk.
Something surreptitious is stealthy, furtive, and often unseemly or unethical: "She takes surreptitious sips from a flask in her bag as she waits in the cold" (Mary V. Dearborn).
Underhand implies unfairness, deceit, or slyness as well as secrecy: The politician achieved success by underhand methods.