It is astonishing to me how many people confuse 'simultaneous' with 'instantaneous'. But the two are nothing alike.
(1) 'Instantaneous' means 'occurring in an instant' or 'occurring at an instant'; 'simultaneous' means 'occurring at the same time'.
(2) Instantaneity is not a relation; event E can be instantaneous in itself simply by occurring in an instant (assuming that it's possible for an event to occur in an instant). Simultaneity, however, is a relation: A can't be simultaneous on its own; it has to be simultaneous with something. A and B, however, can be simultaneous.
(3) Events that are simultaneous can last a short time or a long time. For that matter, there's nothing to prevent two events from being simultaneous for all time. This is obviously not so with instantaneous.
I notice the confusion a lot when people are talking about simultaneous causation; they are constantly assuming that 'simultaneous causation' means 'instantaneous causation'. But, naturally, this is not so.
