Definition of patrol:
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; 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
"in France there is an army of patrols to secure her fiscal regulations. -A. Hamilton."
to go the rounds along a chain of sentinels; to traverse a police district or beat; to go the rounds of, as a sentry, guard, or policeman; as, to patrol a frontier; to patrol a beat