All frames of reference with zero acceleration are in a state of constant rectilinear motion (straight line motion) with respect to one another. In such a frame, an object with zero net force acting on it is perceived to move with a constant velocity, or, equivalently, Newton's first law of motion holds. Those are the frames called inertial. Originally, some physicists like Isaac Newton thought that one of those frames was absolute — the one approximated by the fixed stars. However, this is not required for the definition, and it is now known that those stars are in fact moving.