Hi odin1
Firstly, we should remind ourselves that while some eminent scientists insist that, logically, other universes must exist, there is no proof.
Re the definition of universe: We can define universe in two ways: either as including any other universes as part of it, irrespective of their accessibility, or by considering each as a separate universe. Scientists appear to prefer to differentiate between the 'universe', as commonly understood, and 'multiverse', for the purpose of communicating these ideas unambiguously.
"I'm a little confused. In reference to another universe. How do you know when the one we live in ends?"
Confused? You’re not alone.
This universe has no end in the conventional sense of a connecting point with something beyond it, unless we consider the possiblity of inter-universe wormholes (Schwarzschild wormholes). Metaphorically (and reducing the spatial dimensions from three to two for the sake of understanding the concept) it's described as being either like a flat sheet that extends to infinity in all directions or, alternatively, like the surface of a balloon upon which an ant might crawl indefinitely without finding an end.
