Inertial reference frame
From AMS Glossary
inertial reference frame
Within Newtonian mechanics, a reference frame relative to which every point mass not subjected to a net force is unaccelerated.
Within relativistic mechanics, a reference frame is inertial in a (local) region of space and time if every point mass in this region remains in uniform motion. According to the principle of relativity, all the laws of physics have the same form (and contain the same numerical constants) when expressed relative to any inertial reference frame.
Taylor, E. F. and J. A. Wheeler 1966. Spacetime Physics. 9–12.