Aspherical lens
From The Glossary of Digital Photography
A lens designed to incorporate an aspheric lens element, that is, one that is not spherical, in order to reduce spherical aberrations. These lenses are characterized by relatively high speed and detail sharpness because most light rays are focused at a single point no matter what angle they strike the front of the lens element. Using aspheric lens elements reduces the need for additional lens elements. See also aspheric lens
