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.