Relative complement in the context of Lune (mathematics)

In plane geometry, a lune (from Latin luna 'moon') is the concave-convex region bounded by two circular arcs. It has one boundary portion for which the connecting segment of any two nearby points moves outside the region and another boundary portion for which the connecting segment of any two nearby points lies entirely inside the region. A convex-convex region is termed a lens.

Formally, a lune is the relative complement of one disk in another (where they intersect but neither is a subset of the other). Alternatively, if ${\displaystyle A}$ and ${\displaystyle B}$ are disks, then ${\displaystyle A\smallsetminus A\cap B}$ is a lune.