In observational astronomy an Einstein ring is the deformation of the light from a source into a ring through gravitational deflection of the source's light by a lens. This occurs when the source, lens and observer are all aligned.
Gravitational lensing is a result from Albert Einstein's theory of General relativity. Instead of light from a source traveling in a straight line (in three dimensions), it is bent by the presence of a massive body, which distorts spacetime. An Einstein Ring is a special case of gravitational lensing, caused by the exact alignment of the source, lens and observer. This results in a symmetry around the lens, causing a ring-like structure.
