The primary modern meaning for sally port is a secure, controlled entryway, as at a fortification or a prison. The entrance is usually protected in some way, such as with a fixed wall blocking the door which must be circumvented before entering, but which prevents direct enemy fire from a distance. It may include the use of two doors such as with an airlock.In older days a "sally port" was sort of a dock where boats would pick up a ship's crew and ferry them to the vessel, or vice versa, anchored offshore. (British navy, ca. 1600 to 1900), but it is used that way still especially in coastal Great Britain. However there are not many "old" sally ports left.

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.