How are inertial frames defined in Special and General Relativity? In Newtonian physics, an inertial frame is usually defined as one in which N2 holds. Clearly this cannot be the same definition as for SR. In GR an inertial frame is one in which SR holds (I think). However, there is now a distinction between globally and locally inertial, and frames which are apparently accelerating towards each other can still be locally inertial.