American Dictionary of the English Language

POSTERN, noun [Latin post, behind.]

1. Primarily, a back door or gate; a private entrance; hence, any small door or gate.

2. In fortification , a small gate, usually in the angle of the flank of a bastion, or in that of the curtain or near the orillon, descending into the ditch.

POSTERN, adjective Back; being behind; private.