Lens

Webster's Dictionary

(n.) A piece of glass, or other transparent substance, ground with two opposite regular surfaces, either both curved, or one curved and the other plane, and commonly used, either singly or combined, in optical instruments, for changing the direction of rays of light, and thus magnifying objects, or otherwise modifying vision. In practice, the curved surfaces are usually spherical, though rarely cylindrical, or of some other figure.

The Nuttall Encyclopedia

A piece of glass adapted as convex or concave so as to change the direction of the rays of light passing through it and magnify or diminish the apparent size of an object.