 This way of putting the relativity of simultaneity requires a new bit of technical jargon: the notion of a reference frame. For our purposes, a reference frame is nothing more than a coordinate system that is used to identify the same point in space at different times. Someone on a steadily moving train, for instance, will naturally use a reference frame that is different from someone who is standing on the station platform, since it is natural for the person on the train to think of themselves as stationary, while for the person on the platform it seems obvious that they are moving.