I've noticed recently some people struggling with the difference between 'simultaneous' and 'instantaneous', so I thought I would point out how they are different.
(1) Simultaneity is a two-place relation; instantaneity is a one-place predicate. That is, an event may be instantaneous on its own; that means it occurs, by some measure, at an instant, the equivalent of a single point in the measurement. An event can only be simultaneous with something else: A is simultaneous with B. (The relation is reflexive, so we can say that A is simultaneous with A, but we need to fill both places.)
(2) For events to be simultaneous implies that their measurements overlap; sometimes it's used in a strict sense to mean that their measurements overlap exactly, but in either case it's overlap that matters. Because of this, two things can be simultaneous for a short or for a very extended period of time. It is conceptually possible for two things to be simultaneous forever -- all this means is that they share the same clock-measurements for every clock-measurement, so it would just be a matter of whether there are actually things measurable by a clock that have this relation to each other. By definition, nothing can be instantaneous for an extended period of time.
(3) Something would be instantaneous in terms of a single measurement. It's often the case, however, the simultaneity requires coordinating different measurements. (The primary case where you don't have to do this is when we say something is simultaneous with itself, and it's unclear whether there's any other case.) This is sometimes easy, but it is also sometimes not straightforward at all.
(4) It is extremely evident that there are simultaneous things. For instance, I am typing this simultaneously with the fan going above me and the refrigerator running in the next room. This is something we recognize in ordinary experience. I am thinking of what to type simultaneously with typing it -- there's a tiny delay, of course, between the words being thought and their showing up on the screen, but this is irrelevant because the thinking and the typing are two ongoing events that overlap by clock-measurement. It is not evident at all whether anything ever happens instantaneously -- this claim is something that requires a particular theory of time, and if we're speaking strictly, we never in ordinary experience find anything that literally occurs at a single instant, simply because our ordinary ways of experiencing the world are simply not that precise.
