General Relativity is a classical field theory formulated by Albert Einstein in 1915 that describes gravity as the curvature of four-dimensional spacetime in the presence of matter or energy using Riemannian geometry. (TNG: "The Nth Degree")
This theory unified gravity with acceleration. This is shown through an example of an elevator in space, if accelerating upward, the person occupying the elevator feels as if he is in a gravitational field.
The theory was proven in the year 1919 by a photo of a total solar eclipse. The picture showed light bending around the sun, showing gravity is a curvature of spacetime and not the pull force of Newtonian mechanics.