Locus

The locus is the place on a chromosome where a specific gene is located, a kind of address for the gene. The plural is "loci," not "locuses."

Put another way, a locus is any site that has been defined genetically. It may be a gene, or a part of a gene, or a DNA sequence that plays a regulatory role. A locus may be occupied by different sequence forms or alleles.

Sources[edit]

http://www.genome.gov/glossary.cfm?key=locus