One of the alternative versions of a gene at a given location (locus) along a chromosome
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
An allele is one of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent. If the two alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous. Though the term "allele" was originally used to describe variation among genes, it now also refers to variation among non-coding DNA sequences.