Salients, re-entrants and pockets

A salient is a battlefield feature that projects into enemy territory. The salient is surrounded by the enemy on three sides. This makes the troops occupying the salient vulnerable to attack. The enemy's line facing a salient is referred to as a re-entrant (an angle pointing inwards). A deep salient is vulnerable to being "pinched out" across the base. If that happens the defenders are then surrounded. This is what happened at the Battle of the Bulge in World War II. 
In fortifications, a salient is a part of the defense structure that sticks outward.