A frame of reference was a set of coordinates that could be used to determine positions and velocities of objects in that frame.
A story told by a hologram of Doctor Stephen Hawking, he stated that "in that frame of reference, the perihelion of Mercury would have precessed in the opposite direction." Data further explained that "the joke depends on an understanding of the relativistic curvature of space-time. If two non-inertial reference frames are in relative motion…" but he was abruptly interrupted by Sir Isaac Newton. (TNG: "Descent")