adj. 1
Being or kept hidden.
We went down a secret passage.
n. 1
A piece of knowledge that is hidden and intended to be kept hidden.
"Can you keep a secret?" "Yes." "So can I."

Related words

Word family: secrete secretive secretly secretion
Phrases: secret agent secret sauce secret vice top-secret secret government secret history secret ink secret key

Forms

Secret or Hidden?

That is secret which is intentionally hidden.

J. C. Fernald, English Synonyms and Antonyms (1914)

covert We have been sent on a covert mission.
clandestine Sami and Layla began a clandestine relationship.
surreptitious The surreptitious gesture went unnoticed by the rest of the room.
mysterious A mysterious spaceship was flying away from Earth.
occult Sami read books on Satanism and the occult.

Use in printed books, per decade from 1850 to 2019.

4 letters: dark¹, dern¹
5 letters: close¹, inner², privy²
6 letters: hidden², mystic², unseen², covert², occult², latent², veiled¹, enigma³
7 letters: private², covered², unknown², mystery³, retired², privacy³, obscure², cryptic², furtive², arcanum³
8 letters: mystical³, esoteric⁴, secluded³, shrouded², stealthy², abstruse²
9 letters: concealed², underhand, recondite³
10 letters: classified³, mysterious⁴, undercover⁴, unrevealed
11 letters: clandestine³, sequestered³, kabbalistic
12 letters: confidential⁴
13 letters: surreptitious⁴
14 letters: unacknowledged

The small number after each word is its number of syllables (inner² = 2 syllables).