Source : Webster's Revised Unabridged Dictionary (1913)

Postern \Pos"tern\, n. [OF. posterne, posterle, F. poterne, fr.
   L. posterula, fr. posterus coming after. See {Posterior}.]
   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.

            Out at the postern, by the abbey wall. --Shak.

   2. (Fort.) A subterraneous passage communicating between the
      parade and the main ditch, or between the ditches and the
      interior of the outworks. --Mahan.

Postern \Pos"tern\, a.
   Back; being behind; private. ``The postern door.'' --Dryden.

Source : WordNet®

postern
     n : a small gate in the rear of a fort or castle