James Gasik
We don't talk about Pun-Pun
But "instantaneous" is only ever defined in the magic section, for spell durations on page 203, and even then, all it says is it cannot be dispelled because it's magic exists for only an instant.
That doesn't mean that a spell can't be reacted to, because if it did, the rules would call that out specifically. And despite being instantaneous, other instantaneous effects, like counterspell can affect a spell (despite the fact that Dispel Magic, also instantaneous cannot affect a spell, only it's affect).
I'd also like to point out that just because a spell has an instantaneous effect, does not mean all of it's effects occur in an instant. I'd like to direct your attention to Dissonant Whispers. It's effect is instantaneous, but the subsequent reaction to move away from the caster is not- thus it's magic does actually take more time than an instant to resolve.
That doesn't mean that a spell can't be reacted to, because if it did, the rules would call that out specifically. And despite being instantaneous, other instantaneous effects, like counterspell can affect a spell (despite the fact that Dispel Magic, also instantaneous cannot affect a spell, only it's affect).
I'd also like to point out that just because a spell has an instantaneous effect, does not mean all of it's effects occur in an instant. I'd like to direct your attention to Dissonant Whispers. It's effect is instantaneous, but the subsequent reaction to move away from the caster is not- thus it's magic does actually take more time than an instant to resolve.