The Collaborative International Dictionary of English v.0.48:

Patrol \Pa*trol"\, v. i. [imp. & p. p. Patrolled; p. pr. & vb.
   n. Patrolling.] [F. patrouiller, O. & Prov. F. patrouiller
   to paddle, paw about, patrol, fr. patte a paw; cf. D. poot
   paw, G. pfote, and E. pat, v.]
   To go the rounds along a chain of sentinels; to traverse a
   police district or beat.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Patrol \Pa*trol"\ (p[.a]*tr[=o]l"), v. t.
   To go the rounds of, as a sentry, guard, or policeman; as, to
   patrol a frontier; to patrol a beat.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Patrol \Pa*trol"\, n. [F. patrouille, OF. patouille. See
   Patrol, v. i.]
   1. (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.
          [1913 Webster]

   2. Any perambulation of a particular line or district to
      guard it; also, the men thus guarding; as, a customs
      patrol; a fire patrol.
      [1913 Webster]

            In France there is an army of patrols to secure her
            fiscal regulations.                   --A. Hamilton.
      [1913 Webster]

   3. See Boy Scout.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

patrol
    n 1: a detachment used for security or reconnaissance
    2: the activity of going around or through an area at regular
       intervals for security purposes
    3: a group that goes through a region at regular intervals for
       the purpose of security
    v 1: maintain the security of by carrying out a patrol [syn:
         patrol, police]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "patrol":
   Argus, Charley, airplane spotter, beat, course, cover, defend,
   defending, do, fire patrolman, fire warden, fireguard,
   forward observer, go over, guard, guarding, keep guard, keep vigil,
   keep watch, lookout, lookout man, measure, mount guard,
   night watchman, overpass, pass over, pass through, patroller,
   patrolman, perambulate, peregrinate, pererrate, picket, ply,
   police, protect, protecting, protection, range, range over,
   reconnoiter, rounds, roundsman, safeguarding, scour,
   scour the country, scout, sentinel, sentry, spotter, stand guard,
   stand sentinel, sweep, track, transit, travel over, travel through,
   traverse, vedette, vigilance, voyage, watch, watch over, watcher,
   watchfulness, watchkeeper, watchman