locus (n.) Look up locus at
(plural loci), 1715, "place, spot, locality," from Latin locus "a place, spot; appointed place, position; locality, region, country; degree, rank, order; topic, subject," from Old Latin stlocus, literally "where something is placed," from PIE root *st(h)el- "to cause to stand, to place." Used by Latin writers for Greek topos. Mathematical sense by 1750.