Source : Webster's Revised Unabridged Dictionary (1913)

Spotter \Spot"ter\, n.
   One who spots.

Source : WordNet®

spotter
     n 1: a worker employed at a dry-cleaning establishment to remove
          spots
     2: a worker employed to apply spots (as markers or identifiers)
     3: a person employed to watch for something to happen [syn: {lookout},
         {lookout man}, {sentinel}, {sentry}, {watch}, {scout}, {picket}]
     4: someone who is the first to observe something [syn: {finder},
         {discoverer}]