Official Definition of Omniverse

I wonder what it is official definition omniverse in Marvel.
In many forums, definition of Omniverse is very ambiguous.
Some regard it as multiverse, Others define omniverse as all of universe.
So, What is Official Definition of Omniverse in Marvel?

I always thought that it was multiverse that was ambiguous, whereas omniverse specifically meant the collection of every universe.

An old handbook defined it as every real and fictional universe ever, but that was later changed.

-OHOTMU: Alternate Universes(2005)