The theory of relativity, or simply relativity, generally encompasses two theories of Albert Einstein: special relativity and general relativity. (The word relativity can also be used in the context of an older theory, that of Galilean invariance.) Concepts introduced by the theories of relativity include: Measurements of various quantities are relative to the velocities of observers. In particular, space and time can dilate. Spacetime: space and time should be considered together and in relation to each other. The speed of light is nonetheless invariant, the same for all observers. The term 'theory of relativity' was based on the expression 'relative theory' 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.'