a reason or explanation not immediately or generally apparent.
11.
a method, formula, plan, etc., known only to the initiated or the few: the secret of happiness; a trade secret.
12.
a classification assigned to information, a document, etc., considered less vital to security than top-secret but more vital than confidential, and limiting its use to persons who have been cleared, as by various government agencies, as trustworthy to handle such material. Compare classification(def. 5).
(initial capital letter) Liturgy. a variable prayer in the Roman and other Latin liturgies, said inaudibly by the celebrant after the offertory and immediately before the preface.
late 14c. (n.), c.1400 (adj.), from L. secretus "set apart, withdrawn, hidden," originally pp. of secernere "to set apart," from se- "without, apart," prop. on one's own (from PIE *sed-, from base *s(w)e-; see idiom) + cernere "separate" (see
crisis). The verb meaning "to keep secret" (described in OED as "obsolete") is attested from 1590s. Secretive is attested from 1853. Secret agent first recorded 1715; secret service is from 1737; secret weapon is from 1936.