The observable effects on a body in motion.  As velocity increases, time slows down, mass increases and lengths contract.

Einstein discovered that time and space are interconnected. Height, width, length, and time make up the dimensions of space-time. The famous equation E=mc2 is a consequence of this theory.

Theory developed by Einstein which asserts that all motion is relative; there is no privileged frame of reference against which motion can be measured; the speed of light is the same from all referenc [..]

Theory of relativity applied concerning uniform motion. It proposes that the equivalence of mass and energy and differs from Newtonian physics only when speeds approach that of light.

Einstein's laws of physics in the absence of gravity.

(relativity) A theory that (neglecting the effects of gravity) reconciles the principle of relativity with the observation that the speed of light is constant in all frames of reference.