The metaverse has a consistent definition - at least, if you read Snow Crash. It's just not what any company would actually want to bring to the market.

3D game worlds have value, open systems have value, but in completely opposite and contradictory ways. The value is in the entertainment of playing a well-built one, which means there has to be monetization and centralization. In other words, they have "production value". Open systems are outright allergic to that: while you can monetize, say, a webpage; there's no central WWW server, so there's no technical protection (DRM) against you just reuploading someone else's webpages. This doesn't matter so much for the web, because the cost to produce is low and the web has something the game worlds don't: "interconnection value".

A decade and change ago we had Second Life, a 3D game world built like an open system. This meant that instead of just giving you a bunch of levels with characters and game mechanics, they gave you a sandbox and told you to build whatever. They even released the Viewer source under GPL and at one point even promised to do the same with the Simulator server (which never panned out, although they were trialling grid interconnection with IBM at one point). You could build your own entirely independent Grid via OpenSim. At the same time, the game also had a centralized in-game economy based around selling content to build with. Linden Labs' embrace of openness, as incomplete as it was, wound up totally pissing off a lot of long-time users whose content could now be trivially copied, reuploaded, and resold using Free tools. They also had to deal with malicious Viewer forks, which ultimately wound up with them having an approval process for third-party Viewers, walking back some of that openness promise.

(I'm, of course, skipping over all of the other systems that have cropped up before and since - ActiveWorlds, VRChat, and so on. AFAIK none of them had metaverse pretensions to the same level as Second Life did.)

I've heard people claim Roblox is "the metaverse", and they might be close. However, it's far more of a closed system than even SL's open-viewer, closed-grid approach. They wouldn't for a second tolerate, say, someone writing a custom Roblox client or server. They do have an element of the metaverse that nobody likes to talk about, however: copyright infringement. Roblox has plenty of unlicensed fangames and other content that you wouldn't dream of being able to legally ship in a game world normally. This, to me, is one of the core properties of "metaverse-ness"; the ability for content to just exist in the game world without an associated license.

(Epic, you can use the term "metaverse" when I can design my own Fortnite skins and levels. Just being able to play on my PS4 with Xbox players is only a small part of it.)

Someone might bring up Minecraft at this point. This also has some of the aspects of a metaverse - notably, the block-building lets you go crazy with custom content, and it also allows custom skins. Well... mostly. Custom skins are only present in some versions of the game - notably, all Java versions, as well as Windows, iOS, and Android Bedrock. Other versions replace your skin with the default. There's also some licensed skins that are locked to specific versions of the game, that get replaced with the default skin as well. Cross-play lets you join other player's game sessions, but if you want levels to actually move between devices, you need to have a Realms subscription. There are servers, but custom servers are very new (like, in-alpha new) and not allowed on consoles (or at least on Switch - I don't own the Xbox or PS4 versions to test).

So that's another problem with the idea of a metaverse - the current system has way too many barriers in play to make it work. Remember how Sony made your Fortnite account a PS4 exclusive so you couldn't use your lootboxes on Switch? Imagine that, but 2000x worse, if someone actually tried to bring the metaverse to consoles.