Ask HN: Does the term “Metaverse” have a concrete definition?

I'm trying to read behind the marketing and journalistic misuse of the term. Facebook is trying to promote something, but what is it?

Is it a service? Is it a collection of frameworks for data interoperability and persistence? Is it a VR/AR rendering engine? Is it commercial CAD software with VR capabilities? Is it a VR chatroom? A persistent MMORPG game like second life? A new rendering/game engine?

I'm sure there's a real product (or set of products) buried somewhere underneath all the hype and bad news pieces. I just can't find out what it actually is.