1. the fact of being relative; dependence of a quantity or condition on the status of something else.

2. a theory introduced by Albert Einstein, concerned with physical laws (such as those that describe mass, space, motion, and time) as they are formulated by different observers in uniform relative motion with respect to each other. The special relativity theory postulates that all the laws of physics have the same form in all inertial frames of reference. It also postulates that the speed of light is a constant, independent of any relative motion between the light source and the observer. The general relativity theory postulates that in the neighborhood of a given point, it is not possible to distinguish between a gravitational field and an accelerated frame of reference; i.e., gravitational mass and inertial mass are completely equivalent.