A Relativity

Linguistically:

2. The state of being relative to something else.
3. (physics) The principle that the laws of physics should be the same for all observers.
4. (relativity) Either of two theories (special relativity or general relativity) developed by German-American physicist Albert Einstein. Also called Einsteinian relativity.

Astrophysics:

Einstein's theory of relativity, introduced in two parts

the special theory of relativity:

Challenged traditional notions of space and time that had been accepted for centuries. The special theory of relativity forms the basis of this transformative revolution.

The general theory of relativity:

Extends The principles of the special theory to include the effects of gravity and the analysis of accelerating frames of reference.