A *locus* is the set of all points satisfying a given rule.

For example, a circle is the locus of points which are a fixed distance from a fixed point, and an ellipse is the locus of points \(P\) which satisfy \(PA+PB=c\), where \(A\) and \(B\) are fixed points and \(c\) is a constant with \(c>2AB\).

The plural of locus is *loci*.