The Collaborative International Dictionary of English v.0.48:
Locus \Lo"cus\, n.; pl. Loci, & Loca. [L., place. Cf.
Allow, Couch, Lieu, Local.]
1. A place; a locality.
2. (Math.) The line traced by a point which varies its
position according to some determinate law; the surface
described by a point or line that moves according to a
Plane locus, a locus that is a straight line, or a circle.
Solid locus, a locus that is one of the conic sections.