hidden (adj.)

past-participle adjective from hide (v.1); a Middle English formation (Old English had gehydd "hidden") on the model of ride/ridden, etc. As "secret, occult" from 1540s. Hidden persuaders (1957) was Vance Packard's term for ad men.

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Definitions of hidden from WordNet

hidden (adj.)
not accessible to view;
concealed (or hidden) damage
Synonyms: concealed / out of sight
hidden (adj.)
designed to elude detection;
a hidden room or place of concealment such as a priest hole
Synonyms: secret
hidden (adj.)
difficult to find;
hidden valleys
a hidden cave
Synonyms: obscure