lens

Definition: A curved, ground and polished piece of glass or other transparent material used for the refraction of light. A perfect converging lens brings the rays of a parallel beam to a real point called the real principle focus. A diverging lens causes the rays of a parallel beam to diverge, as though they came from a “virtual” point behind the lens, called the virtual principle focus.

Imperfectly constructed lenses (or systems of lenses and mirrors) can suffer from aberrations.