Noun, pl. alleles
1. One of a number of alternative forms of the same gene or same genetic locus (generally a group of genes). It is the alternative form of a gene for a character producing different effects. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. However, many variations at the genetic level result in little or no observable variation. (wikipedia.org)
2. One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome (Dictionary.com)
One member of a pair (or any of the series) of genes occupying a specific spot on a chromosome (called locus) that controls the same trait.
Word origin: From German Allel, shortened from Allelomorph, from English allelomorph.