# Locus

A Locus is a collection of points that share a property. Often a figure or curve in the plane (or other space) will be defined as a locus. For example, the circle is the collection of points at a given distance (the radius) from a given point (the centre of the circle). Similarly, an ellipse can be defined as the collection of points whose sum of distances to the foci $F_0$ and $F_1$ is constant.