Einstein's principle of relativity

The laws of physics have the same form in all inertial frames; uniform absolute motion cannot be identified by experiments confined to an inertial laboratory. The invariant vacuum speed of light is a separate postulate. Together they motivate the Lorentz transformations of special relativity.

Einstein synchronisation sets clocks at rest in one inertial frame using exchanged light signals. If the sending clock records departure at $t_A$ and return at $t_A^{\prime }$, the distant clock's reflection reading is set to $t_B=(t_A+t_A^{\prime })/2$. This convention assigns equal outgoing and incoming travel times and defines the frame's distant simultaneity. It does not set the arrival time of an image equal to the time of the remote spacetime event.