Salients, re-entrants and pockets
A salient is a battlefield feature that projects into enemy territory. The attacking troops creating the salient are vulnerable to attack from the front and flanks of the salient. 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 attackers are then surrounded.* This is what happened at the Battle of the Bulge in World War II.