1) One of a number of alternative forms of a gene. Commonly only two alternative forms exist for a gene.
(Adapted from Edexcel GCSE SCience and Data Resource Book, UG005206) 

2) 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

» Biology Glossary for 11-16 Yr Olds (Chad Evans)