- A reference frame in which the observers are not subject to any accelerating force. In Special Relativity, time measurements in inertial frames that are not at rest with respect to each other are not equivalent; each inertial frame must have its own time coordinate, the value of which is the time as read off a standard clock at rest in that frame. Also called inertial frame of reference inertial reference frame, inertial system Compare non-inertial frame. See also Special Relativity.
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