Is the universe really a 'multiverse'?

Some physicists seem to strongly believe that this is the case, but the major problem is that there is no practical way to test such an incredible idea. We don't really know...it all depends on how you define the term.

All that we can ever have direct knowledge about lies within our visible horizon, which encompasses a portion of the space that was produced at the big bang. Our particular universe might be defined as all the matter and energy that came out of the big bang which we see in our past. According to Inflationary cosmology, our universe emerged from a patch of quantum instability in a larger...something...These patches could be strikingly different kinds of space and matter than what we see around us, but in the distant future, our expanding horizon will eventually reach them and so we will begin to see these distant 'patches' for the first time.

In Brane Theory, our universe is a 3-D brane floating in an 11-dimensional arena where gravity shows its true colors as a strong force. There are presumably other brane-universes floating 'out there' too!

I don't know whether you would call these 'other universes' or not. I think at some point it becomes a matter of semantics...and science fiction!