Patrol is a command in StarCraft and StarCraft II, available to combat units in StarCraft and mobile units in StarCraft II. To use patrol, press "P" then left click a point on the map. The unit will patrol from its original location to the point of the map and back. In StarCraft II, a patrol can contain multiple waypoints, giving a more complicated path.
Units patrol aggressively, attacking whatever enemy unit approaches, but then retreating to their original line once there are no enemy units present.
In StarCraft, patrols often "fell apart" due to coding restrictions, with units gradually falling into different patterns than the one given.