Source : Webster's Revised Unabridged Dictionary (1913)

Watcher \Watch"er\ (-[~e]r), n.
   One who watches; one who sits up or continues; a diligent
   observer; specifically, one who attends upon the sick during
   the night.

Source : WordNet®

watcher
     n 1: a close observer; someone who looks at something (such as an
          exhibition of some kind); "the spectators applauded the
          performance"; "television viewers"; "sky watchers
          discovered a new star" [syn: {spectator}, {witness}, {viewer},
           {looker}]
     2: a guard who keeps watch [syn: {watchman}, {security guard}]
     3: a person who keeps a devotional vigil by a sick bed or by a
        dead body