Locus (from Latin locus `place, region') is a set of points satisfying a certain condition and such that no point outside the set satisfies this condition.

For example, in the plane the locus of such point that is at a fixed distance $r$ from a fixed point $O$ is the circle with centre $O$ and radius $r$ the locus of such point which is equidistant from both ends of a line segment is the perpendicular bisector of this line segment.

In general, the loci in the plane are geometric curves; thus one may speak of locus curves.