The term 'parallel universe' is used for a Plane that seems to be an almost exact copy of another Plane - that is, it shares the same natural (and supernatural) laws, Gods, afterlife and so on.
In a few cases, a parallel universe may be so similar to another Plane that they are even inhabited by identical individuals and may share the same history. In the majority of cases however, a parallel universe will differ in terms of its history and inhabitants.
Not all Planes have a parallel universe, but some have a large number of them.