What does aspheric mean?

An aspheric lens is a magnifying lens with a surface profile that is neither a portion of a sphere nor of a circular cylinder. This more complex surface can eliminate spherical aberrations as well as other optical aberrations. The aspheric lens is smaller and lighter than multi-lens magnifiers or the convex lenses. The thinner lens results in a smaller and lighter magnifier. Thin optical lenses used in eyeglasses sometimes use thin aspheric lenses to eliminate the visual distortion.  more