We now need to be more precise about some of the terms that we will be using.
Observers are simply people or instruments capable of making and recording measurements.
Books on Special Relativity often refer to frames of reference . A frame of reference can be thought of as a set of three orthogonal meter sticks, with a clock attached. The meter sticks provide coordinates with respect to which we can measure the position of objects or events, and hence their displacements as they move. The clock is needed to measure time, so that we can measure velocities and accelerations. Of course, such a frame of reference is useless unless it also holds an observer.