The Collaborative International Dictionary of English v.0.48:

allele \al*lele"\ n.
   1. 1

   1. one of two or more alternative forms of a gene that can
      have the same place on homologous chromosomes and are
      responsible for alternative traits.

   Syn: allelomorph
        [WordNet 1.5]

   2. either of a pair of Mendelian characters that may occur in
      an organism as a consequence of variation at one gene
      locus.
      [PJC]

WordNet (r) 3.0 (2006):

allele
    n 1: (genetics) either of a pair (or series) of alternative
         forms of a gene that can occupy the same locus on a
         particular chromosome and that control the same character;
         "some alleles are dominant over others" [syn: allele,
         allelomorph]