a massive celestial body, esp. a galaxy or cluster of galaxies, having enough gravity to refract light waves from a more distant object, esp. a quasar, so that an observer sees an amplified or multiple image ()
See gravitational lens in American Heritage Dictionary 4
A massive celestial object, such as a galaxy, whose gravity bends and focuses the light of a more distant object, resulting in a magnified, distorted, or multiple image of the original light source for a distant observer.