Wiktionary
n. (context relativity English) The principle that only the motion of objects relative to one another can be measured, and that there is no absolute frame of reference.
WordNet
n. (physics) a universal law that states that the laws of mechanics are not affected by a uniform rectilinear motion of the system of coordinates to which they are referred
Wikipedia
In physics, the principle of relativity is the requirement that the equations describing the laws of physics have the same form in all admissible frames of reference.
For example, in the framework of special relativity the Maxwell equations have the same form in all inertial frames of reference. In the framework of general relativity the Maxwell equations or the Einstein field equations have the same form in arbitrary frames of reference.
Several principles of relativity have been successfully applied throughout science, whether implicitly (as in Newtonian mechanics) or explicitly (as in Albert Einstein's special relativity and general relativity).
Usage examples of "principle of relativity".
If the principle of relativity were not valid we should therefore expect that the direction of motion of the earth at any moment would enter into the laws of nature, and also that physical systems in their behaviour would be dependent on the orientation in space with respect to the earth.
Lord Isaac Newton had discovered his Principle of Relativity in the seventeenth century.
And by the principle of relativity, this must be true not only for light clocks but also for any clock—.