Game world (also Ludic World) – used in contrast to ‘Social World’ under the super-set of Virtual Worlds. i.e. those with a game mechanic built into the code, generally from the DikuMUD strain of design

Metaverse – Currently has at least three uses (ultimately derived from N Stephenson, of course – but the book meaning is not that which it presently tends to denote)
1. The 3D web of the future built, possibly, on some of the Social World technology we have now.
2. The general set of Virtual Worlds implicitly of the present dominant form.
3. In relation to the ‘Thin Virtual Worlds’ that are web based, a term to explicitly denote the current mode of virtual world be it either Social or Game.

MMORPG – (Massively Multiplayer On Line Role Play Game) A term that seems to be dying out. Pronunciation varies from M.M.O.R.P.G. to ‘More-pig’. Generally means the same as Game World (though not generally used in contrast to Social World in the same way) but is sometimes used as a super-set in a similar way to ‘virtual world’ when used in that mode.

Social World – used in relation to Game World and sometimes MMORPG or MMO under the super-set of Virtual Worlds. Denotes a virtual space either with no game mechanic in the code or at least no over-aching game mechanic e.g. where there is a social space with embedded mini-games.

Virtual World – this I have found now has two uses (which is what got me thinking about all this), these are relational.
1. When used in relation to Social Worlds and Game Worlds, Virtual World is the super-set of these, generally following Bartle’s technical definition.
2. When used in relation just to MMO’s or Game Worlds the term is synonymous with Social worlds.