Postern

n. [O. Fr. posterne, N. Fr. pōterne, from Lat. post, after, behind; L. Lat. posterna, posterula, O. It. postierla, Pr. posterllu.] 1. Originally, a back door or gate; a private entrance; hence, any small door or gate. He by a privy postern took his flight. Spenser. 2. (Fort.) A subterraneous passage between the parade and the main ditch, or between the ditches of the interior of the outworks. Mahan.

a. Back; being behind; private. Dryden.