Good question. It stands for Multi-user Object-oriented Shared Sandbox. But what is it?

Multi-user text games split early into two branches that barely touched for years.

The combat lineage: MUD1 –> AberMUD –> DikuMUD / LPMud. Goal-driven RPG worlds: levels, monsters, combat, loot, areas. Here the world is built offline: DikuMUD loads area files written outside the game; on LPMud, wizards write code in their own directories. There's no in-game way to carve out a new room. This is what most people mean when they say “a MUD.”

The social / creative lineage: TinyMUD –> MUSH, MUCK, MOO. Built around presence, conversation, and – the radical part – building the world live while it runs.

If the signature of the MUD is the fantasy setting, combat, level advancement and sense of adventure/heroism, then the signature of the second kind is the dig command.

The dig command was introduced by TinyMUD (Jim Aspnes, 1989).

It carried a genuinely new idea: a player, standing inside the running world, types a word and a new room springs into being; already wired to where they're standing. Building stopped being a chore done by staff with the server offline, and became something you do live, in play. Everything in the MUSH/MUCK/MOO family inherits it.

dig was adopoted by every kind of world-building MUD after TinyMUD. I didn't know that when I wrote it, and I have never actually played a MUCK, MUSH or MOO before – but, it makes a lot of sense as a command name, doesn't it? It's the defining gesture of an entire branch of the family; and the branch MOSS belongs to, technically. But I think you will find our dig command quite special compared to others.

Within that branch, building is common to all three. What actually separates them is the in-world programming language: whether, and how, ordinary users write behaviour into the world:

• MUSH (PennMUSH, TinyMUX): “softcode”: attributes plus a text-rewriting language, with flags and locks. Heavily role-play oriented.
• MUCK (FuzzBall): MUF, a Forth-like language living on objects.
• MOO (LambdaMOO, 1990): fully object-oriented: every thing is an object with properties and verbs, inheriting from parent objects, and players program the world live. The “OO” is the whole point.

My experience is with the classic MUD. JediMUD. CircleMUD. Medievia. But my idea has a lot in common with the MUCK/MUSH/MOO.

MOSS can dig rooms and shape zones live (see How to Build) just like a TinyMUD. But we don't have an in-world programming language, and the spirit of the world will become remarkably Diku (that's just how I envision a MUD).

That combination of TinyMUD's live, in-world building bolted onto Diku's structured areas, and combat/character development is unusual, and arguably the most interesting thing about a MOSS.

A few MOSS paradigms won't find in the classics:

• Draft / commit: a dug room starts as a draft you can walk around in, and becomes permanent only when you commit it. A staging step the old systems never had.
• A browser, not telnet: you play MOSS over the web, not a 1990s terminal.

The name promises “Object-oriented,” and that part is still ahead of us. Today there are rooms and zones, but no objects, nothing to pick up, no items with behaviour. The next step is objects and inventories; the step after that, if MOSS ever wants to lean towards a true MOO, is verbs on objects: a small in-world language so builders can give the world behaviour, not just shape. I think that is far off, but I do have some ideas on how to do it. Honestly though? Never played a MOO.

It's the thing you just read about: a world you build from the inside, arranged into areas, with combat and character development, played in your browser, slowly in size and scope – a bit like moss. In some ways it's an unfair question. MUD, MUCK, MUSH, MOO – each defined itself by what it did, how people experienced it.

So, what's a moss? Go to How to Play and explore! Because as Ensign Harry Kim once said, “When I think about everything we've been through together, maybe it's not the destination that matters, maybe it's the journey, and if that journey takes a little longer, so we can do something we all believe in, I can't think of any place I'd rather be or any people I'd rather be with.”