The Collaborative International Dictionary of English v.0.48:

crossover \crossover\ n.
   1. (genetics) the interchange of sections between pairing
      homologous chromosomes during the prophase of meiosis.

   Syn: crossing over.
        [WordNet 1.5]

   2. a structure, such as a bridge, bearing a path for crossing
      over a river or highway.
      [PJC]

   3. votes from members of one political party cast for
      candidates of another party; -- called also crossover
      vote; as, there was a high crossover in this election.
      [PJC]

   4. a member of one political party who votes for a candidate
      of another party; -- called also crossover voter.
      [PJC]

   5. (Plumbing) a U-shaped section of pipe which serves to pass
      one pipeline over another which is directly in its path.
      [PJC]

WordNet (r) 3.0 (2006):

crossover voter
    n 1: a voter who is registered as a member of one political
         party but who votes in the primary of another party [syn:
         crossover voter, crossover]