"Multiverse" is correct ... As "universe" means there is only one, so there can't be parallel universes. There can be parallel multiverses, but that is redundant phrasing. Unless there are intersecting multiverses.
Not really a simple answer for this one.
These quotes come from the physic stack exchange and from
In string theory, the multiverse is a theory in which our universe is not the only one; many universes exist parallel to each other. These distinct universes within the multiverse theory are called parallel universes.
“Parallel universes “ is a description of the many worlds interpretation of quantum mechanics.
Your still learning to walk,
someday you'll fly.
Remember, there has to be a beginning,
and an ending so it can start all over again.