Relativity postulates that all frames of reference are equivalent if and only if they are not accelerated.
In the physics of general relativity, the equivalence principle refers to several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein’s assertion that the gravitational “force” as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.
All non-inertial frames are accelerating with respect to all inertial frames.