1. Everything we know about anything and everything that we have been exposed to that is in existence. Anything that we don't know or have not been exposed to is not part of our universe, and could be considered 'nonexistence' or 'nothing'.
Stating that there are multiple universes that exist, or a 'multiverse' violates the actual definition of a universe.
If we know about other universes, they automatically become a part of our commutative universe, or a part of the overall knowledge we have of everything and anything in existence.
2. The universe is everything that is currently present in the reality or realm of existence. It is everything that has existed in the past, and everything that will exist in the future.
3. It is an accumulation of our observable space-time and can only be defined based upon our reality and the three dimensional* space-time structure that we observe and experience.
*Even if other dimensions were present in space, we cannot define them as part of the universe because we do not directly interact with them, and cannot be affected by their presence. Some suggest that it may be impossible for our reality (our observable area of space-time) to interact with the most distant regions of space (the place where these additional dimensions may occur). This is caused by light's theorized inability to travel faster than the expansion of the universe. Because we cannot interact with other dimensions, we cannot consider them part of our universe.
Note: All of these definitions of 'universe' are intended to go hand-in-hand with one another. They all are suggested to be simultaneously true.
1. Spacetime, we know that spacetime exists. It is part of our universe. It is expanding. We do not know what it is expanding into. It is expanding into 'nonexistence'.
2. Benjamin Franklin existed, and there is then light that contains his image somewhere in the universe. He is a part of the universe. The sun will rise tomorrow morning, and that moment in spacetime is part of our universe as well, even though the light has not reached us or been created yet.
3. I can move my hand through 3 evident dimensions of space without being effected by any additional dimensions in my reality, therefore these three dimensions are part of the universe, and any additional undetectable, theorized dimensions are considered 'nonexistent' because I do not observe them nor do I know about them.