1. It is often defined as everything that is, ever was, and ever will be. This describes a 4-dimensional universe, but by adding every possibility to the definition it describes a 5-dimensional universe, which is consistent with multipathism.
2. The definition of universe is sometimes limited to only everything we humans can observe - which is about 80 billion galaxies.
3. Still another definition proposes that we each have personal universes, which are limited to what you observe at any one time. This is only a 3-dimensional universe, but the definition can easily be expanded into the 4th and 5th dimension by including time and/or all paths that lead from you and to you. These universes end up looking like light cones, centered around the person in question. In this view the universe is relative to the observer.
4. Science and String Theory are tending towards a formal definition of the universe as an n-dimensional brane in a larger, hypothetical, multiverse.
On this site I use the term universe to refer to either #1 or #3, allowing context to determine the difference. I call #2 the observable universe, and #4 the crazy brane.