The word locus (plural loci) is Latin for "place".

• In biology, a locus describes the position of a gene (or other significant sequence) on a chromosome. A locus can be occupied by any of the alleles of the gene. Diploid or polyploid cells are either homozygous (have the same allele at a locus) or heterozygous (have different alleles at a locus).
• In mathematics, a locus is the set of points satisfying a particular condition. It is mostly used when the set of points forms a curve of some sort.
• In the classics, literature or theology, locus (or locus classicus) marks the first appearance of a phrase or the definitive passage that is authoritative for an idea.