The Collaborative International Dictionary of English v.0.48:

Dancer \Dan"cer\, n.
   One who dances or who practices dancing.
   [1913 Webster]

   The merry dancers, beams of the northern lights when they
      rise and fall alternately without any considerable change
      of length. See Aurora borealis, under Aurora.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dancer
    n 1: a performer who dances professionally [syn: dancer,
         professional dancer, terpsichorean]
    2: a person who participates in a social gathering arranged for
       dancing (as a ball) [syn: dancer, social dancer]