# Locus

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.