Question

Two events are defined to be simultaneous if an observer measures them as occurring at what?

Answer 1

same time

28.2 Simultaneity And Time Dilation  Two events are defined to be simultaneous if an observer measures them as occurring at the same time. They are not necessarily simultaneous to all observerssimultaneity is not absolute.  Time dilation is the phenomenon of time passing slower for an observer who is moving relative to another observer.  Observers moving at a relative velocity v do not measure the same elapsed time for an event. Proper time t 0 is the time measured by an observer at rest relative to the event being observed. Proper time is related to the time by an Earth-bound observer by the equation.