A massive object whose gravitational field redirects or bends light rays.
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 (gravitational lensing)
A massive celestial object, such as a galaxy, whose gravity can act as a lens that functions to bend and focus the light of a more distant object. This results in a magnified, distorted, or multiple image of the original light source for a distant observer.