a Cartesian frame in which free particles move without acceleration, in accordance with Newton's first law of motion
a coordinate system defined by the non-accelerated motion of objects with a common direction and speed (as opposed to a non-inertial reference frame)
a reference frame in which Newton's first law of motion holds
a hypothetical frame of reference whose motion, if any, is utterly straight, steady and unaltered by external forces. Because gravity is everywhere, such an ideal frame of reference can exist only in the mind.
