frame of reference

A frame of reference is the imaginary platform of clocks and measuring rods from which you describe where things are and how they move. To say a ball is 'at three metres, moving east at two metres per second' you must first agree on an origin, a set of directions, and a way to read off time. That whole apparatus, real or imagined, is the frame.

The key idea is that there is no such thing as motion 'in itself' — motion is always motion relative to some chosen frame. A passenger asleep on a train is at rest in the train's frame yet hurtling along in the frame of the platform. Neither description is more correct; they are simply two honest answers to two different questions.

Because every measurement of position, velocity, and even time is made within some frame, comparing frames is the central business of relativity. Galileo, Newton, and Einstein all start by asking how the very same event looks when described from a frame that is moving relative to yours. The surprises of relativity come from how those descriptions must be stitched together.