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 given law.

Plane locus, a locus that is a straight line, or a circle.

Solid locus, a locus that is one of the conic sections.

Source: Webster's Revised Unabridged Dictionary (1913)