WordDictionary

Noun:

• Any perambulation of a particular line or district to guard it; also, the men thus guarding; as, a customs patrol; a fire patrol.
• military 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.
• military 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.
• military The guard or men who go the rounds for observation; a detachment whose duty it is to patrol.

Verb:

• intransitive To go the rounds along a chain of sentinels; to traverse a police district or beat.
• transitive To go the rounds of, as a sentry, guard, or policeman; as, to patrol a frontier; to patrol a beat.