The Collaborative International Dictionary of English v.0.48:

Watchman \Watch"man\, n.; pl. Watchmen.
   [1913 Webster]
   1. One set to watch; a person who keeps guard; a guard; a
      sentinel.
      [1913 Webster]

   2. Specifically, one who guards a building, or the streets of
      a city, by night.
      [1913 Webster]

   Watchman beetle (Zool.), the European dor.

   Watchman's clock, a watchman's detector in which the
      apparatus for recording the times of visiting several
      stations is contained within a single clock.

   Watchman's detector, or Watchman's time detector, an
      apparatus for recording the time when a watchman visits a
      station on his rounds.

   Watchman's rattle, an instrument having at the end of a
      handle a revolving arm, which, by the action of a strong
      spring upon cogs, produces, when in motion, a loud, harsh,
      rattling sound.
      [1913 Webster]

WordNet (r) 3.0 (2006):

watchman
    n 1: a guard who keeps watch [syn: watchman, watcher,
         security guard]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "watchman":
   Argus, Charley, airplane spotter, fire patrolman, fire warden,
   fireguard, forward observer, lookout, lookout man, night watchman,
   patrol, patroller, patrolman, picket, roundsman, scout, sentinel,
   sentry, spotter, vedette, ward, watch, watcher, watchkeeper



Bouvier's Law Dictionary, Revised 6th Ed (1856):

WATCHMAN. An officer in many cities and towns, whose duty it is to watch
during the night and take care of the property of the inhabitants.
     2. He possesses generally the common law authority of a constable
(q.v.) to make arrests, where there is reasonable ground to suspect a
felony, though there is no proof of a felony having been committed. 1 Chit.
Cr. L. 24; 2 Hale, 96; Hawk. B. 2, c. 13, s. 1, &c.; 1 East, P. C. 303; 2
Inst. 52; Com. Dig. Imprisonment, H 4; Dane's Ab. Index, h.t.; 3 Taunt. R.
14; 1 B. & A. 227; Peake, R. 89; 1 Moody's Cr. Cas. 334; 1 Esp. R. 294; and
vide Peace.
     3. By an act of congress, approved Sept. 30, 1850, the compensation of
watchmen in the various departments of government, shall be five hundred
dollars per annum.