Locus

Locus, in Geometry, denotes the line or surface traversed by a point which is constrained to move in accordance with certain determinate conditions. Thus, the locus of a point which must always preserve the same uniform distance from a fixed point is the surface of a sphere; but if the motion be at the same time confined to a plane, the locus then will be a circle: this is an illustration of the division into solid and plane loci which prevailed among the ancients. In modern Geometry plane loci are treated under the name of Curves (q.v.).