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]