The rules relating observations in one inertial frame of reference to the observations of the same phenomenon in another inertial frame of reference. Casually applied only to the Einsteinian Special Theory of Relativity, but a more general term.
Relativity, Special: The specific set of rules relating observations in one inertial frame of reference to the observations of the same phenomenon in another inertial frame of reference. Einstein's theory of special relativity ensures that the laws of physics for mechanicsl and for electromagnetism are the same for all such observers. It postulates that the speed of light is the same for all observers. One of its more famous consequences is the equivalence of matter and energy through the equation E = mc 2.
See also General Relativity.