Relativity

Galileo first developed the principle of relativity, being the postulate that the laws of physics should take the same form for all observers in uniform motion with respect to each other. It incorporates the classical view that time is universally the same for all such observers. The same principle holds in Newtonian mechanics.

Einstein's theory of relativity consists of special relativity and general relativity. It reshaped the classical view, arguing that time is relative to the observer, in an analogous way to position in space. There is ample experimental evidence that this is indeed the case, however paradoxical the implications may appear.

General relativity has a unique role amongst physical theories in the sense that it interprets the gravitational field as a geometric phenomenon. More specifically, it assumes that any object which possesses mass will curve the 'space' in which it exists, this curvature being equated to gravity. Exerpt from Wikipedia. Information held under GNU Free Documentation License.