Relativity, in the context of physics, refers to the theory of relativity, which encompasses two major theories: special relativity and general relativity.
Special relativity, formulated by Albert Einstein in 1905, is a theory that describes the behavior of space and time in the absence of gravitational forces. It introduces the concept that the laws of physics are the same for all observers in inertial (non-accelerating) reference frames. The theory postulates that the speed of light in a vacuum is constant and that the laws of physics should remain unchanged under transformations between inertial frames of reference. Special relativity deals with phenomena involving relative motion, particularly at speeds approaching the speed of light.
General relativity, developed by Einstein in 1915, is a theory that provides a description of gravity as a curvature of spacetime caused by the presence of mass and energy. In general relativity, the principles of special relativity are extended to include accelerated motion and the effects of gravity. General relativity describes how objects with mass or energy curve the fabric of spacetime, causing other objects to move on curved paths. It explains phenomena such as the bending of light around massive objects and the expansion of the universe.
So, while relativity theory does involve the study of velocities, it is not limited to that aspect alone. It encompasses a broader understanding of the behavior of space, time, and gravity, and how they interact with matter and energy.