What does "patrol" mean?

Patrol: (Mil.) (a) A going of the rounds along the chain of sentinels and between the posts, by a guard, usually consisting of three or four men, to insure greater security from attacks on the outposts. (b) A movement, by a small body of troops beyond the line of outposts, to explore the country and gain intelligence of the enemy's whereabouts. (c) The guard or men who go the rounds for observation; a detachment whose duty it is to patrol.