What is the definition of an inertial frame ? I've read that Inertial frames are reference frames in which newton's first law applies (i.e.bodies subject to zero net external force moves at constant velocity) , however newton's 1st law itself is only valid under inertial frames. I find it weird because that would make inertial frames circularly-defined. some texts define it without direct reference to "Newton's 1st law", defining as "frames in which an object subject to no net external force moves in constant velocity" , but I don't think that addresses the issue at all. Are there any alternative definitions?