The traditional definition of universe is something like
"The Universe is all of spacetime and everything that exists therein".
But in multiverse theory one talks about "universes" and this raises the question of how one can in principle distinguish between two different universes.
A similar question is asked here and the top answer says "In addition, as can be shown by some holographic arguments, it is not really meaningful to talk about things that could exist outside our own cosmic horizon (or universe)." But here is an article in the Quanta Magazine which says "If the universe that we inhabit had long ago collided with another universe, the crash would have left an imprint on the cosmic microwave background (CMB)"
So it seems that at least in some multiverse theories, different universes can send and receive physical information (light or gravitational force) between each other. So the question is
If light or gravitational force can travel between two universes then how would you know both are not part of the same universe (other than by a priori assumption)?