The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity.The basic overall concept is that both time and space are relative, not fixed. However, the word relativity is sometimes used in reference to Galilean invariance.

The term "theory of relativity" was based on the expression "relative theory" (German: Relativtheorie) used by Max Planck in 1906, who emphasized how the theory uses the principle of relativity. In the discussion section of the same paper Alfred Bucherer used for the first time the expression "theory of relativity" (German: Relativitätstheorie).