Locus

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.