SyFy Wire host Jackie Jennings invited two other versions of herself to help explain the difference between a multiverse and a parallel universe. When that didn’t prove fruitful, she turned to Avengers: Endgame and the trailer for Spider-Man: Far From Home. Essentially, a parallel universe is one entity with multiple timelines, while a multiverse contains separate, distinct entities that may have their own timelines. Jennings explains it better using trees.
Think of it like a tree. a tree is one universe and there are all these different branches that are still part of the same tree. Those are alternate timelines…so one universe, multiple timelines, same tree. A multiverse would be like a forest – a billion different trees each one with its own universe and its own set of timelines.
— SYFY WIRE (@SYFYWIRE) May 15, 2019