"our universe is one of potentially numerous 'slabs' floating in a higher dimensional space, much like a slice of bread within a grander cosmic loaf" ~Brian Greene~ Columbia University Physicist
In our world, we recognize three dimensions of space and one dimension of time, parallel universes comes from the idea that there are many more dimensions to our world. If other three dimensional membranes of space exist in parallel to our own three dimensional membrane of space, they to would be considered their own "Universes".