locus - n.
(pl. loci)
1 a position or point, esp. in a text, treatise, etc.
2 Math. a curve etc. formed by all the points satisfying a particular equation of the relation between coordinates, or by a point, line, or surface moving according to mathematically defined conditions.
3 Biol. the position of a gene, mutation, etc. on a chromosome. [L, = place]