Max Tegmark covers this in Our Mathematical Universe.

It's a question of definitions. He makes the argument that defining "universe" to mean "our visible universe", that is, a radius of around 14B ly, is the most useful; if you assume infinite space and infinite matter (which seem to follow from the theory of inflation, which has provided explanations and predictions elsewhere), there are likely infinitely many universes all around following this definition (for other visibility scopes).

Just an unnecessarily pendantic note: the visible (observable) universe would be defined as anything that emitted radiation that would have reached us, and in an expanding universe that means it's considerably larger than 14-ish billion light years radius. Conveniently, it works out to about 14 billion parsecs, so nobody needs to remember another number, they just need to do the conversion.

Wouldn’t it be fair to consider a causally disconnected (relative to us) place to be another universe, at least in layman terms? After all, it is expected in a couple trillion years that nothing outside the local group will be observable.

Who knows how much “disconnected universe” there is. Unless that is somehow known, which seems difficult since it is by definition causally disconnected.

Max Tegmark covers this in Our Mathematical Universe.

It's a question of definitions. He makes the argument that defining "universe" to mean "our visible universe", that is, a radius of around 140B ly, is the most useful; if you assume infinite space and infinite matter (which seem to follow from the theory of inflation, which has provided explanations and predictions elsewhere), there are likely infinitely many universes all around following this definition (for other visibility scopes).

This was certainly true when I was a kid, and "universe" meant "all existing matter and space (observable or not)".

As a youtube-couch expert on astrophysics, I hear "universe" redefined as something like "a bubble of matter and spacetime that is 'quarantined' and can never communicate with the rest of the cosmos" whenever multiverse theory is discussed.