The word port is ultimately from Latin porta for door. Often the term postern is used synonymously. It can also mean an underground tunnel, or passage (i.e. a secret exit for those besieged). A sally, ultimately derived from Latin salīre (to jump), or sortie, is a military maneuver, typically during a siege, made by a defending force to harass isolated or vulnerable attackers before retreating back behind their defenses. Sallies are a common way for besieged forces to reduce the strength and preparedness of a besieging army; a sally port is therefore essentially a door in a castle or city wall that allows troops to make sallies without compromising the defensive strength of the fortifications.