Lo·cus n.; pl. Loci & Loca
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.