Satu dari dua atau lebih bentuk alteratif gen yang menempati lokasi sesuai pada kromosom hemolog; yang menentukan sifat alternatif yang diturunkan. Kata sifat: allelic.

Merriam-Webster Online Dictionary
allele (noun)
any of the alternative forms of a gene that may occur at a given locus
either of a pair of alternative Mendelian characters (as smooth and wrinkled seed in the pea)
Allele (Wikipedia)

An allele (/əˈliːl/) is the variant form of a given gene. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. However, most genetic variations result in little or no observable variation.

The word "allele" is a short form of allelomorph ("other form", a word coined by William Bateson), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes. It derives from the Greek prefix ἀλλήλ, allel, meaning "reciprocal" or "each other", which itself is related to the Greek adjective ἄλλος (allos; cognate with Latin "alius"), meaning "other".

Most multicellular organisms have two sets of chromosomes; that is, they are diploid. These chromosomes are referred to as homologous chromosomes. If both alleles at a gene (or locus) on the homologous chromosomes are the same, they and the organism are homozygous with respect to that gene (or locus). If the alleles are different, they and the organism are heterozygous with respect to that gene.