Dungeon Master. Game Master. MC. Referee. Storyteller. Facilitator.
Not every game has a player who is in charge, or first among equals, but many games do. Some games have what looks like a GM role, but isn't. What follows is my experience and opinion. If you disagree and can explain why in detail, I'd love to hear it.
I feel that this type of role can be broadly characterized by two measurements: Authority (how much net power this player has over other players, their characters, and the world) and Adversity (to what degree this player ought to be challenging the other players' characters' goals).
Dungeons & Dragons
The AD&D Player's Guide and DMG describe the role as "campaign referee" or "Dungeon Master". The preface to the DMG vests creative authority in the DM's hands. They are owner and arbiter of the world. The 3rd edition largely echoes this advice, adding "determine play style" and "teach the game" as responsibilities. The 4th edition DMG describes the DM as "rules moderator, narrator, player of many different characters, and primary creator of the game's world, the campaign, and the adventure". The 5th edition DMG goes into a little more detail: "the DM creates adventures by placing monsters, traps, and treasures for the other players' characters to discover". A summary of the D&D philosophy is also found here: "You're the DM, and you are in charge of the game".
All of these things add up to both high Authority and Adversity. The GM can override the rules, make decisions in the name of fun, and guide the flow of the game. However, they are also expected to drop challenges in the PCs' paths, and early D&D especially feels like it emphasizes an atmosphere of (friendly) competition. To be clear: I do not feel like the goal of the DM is "kill the PCs". That said....
The OSR (variously Old School RPGs or other terms) is a modern movement espousing a return to form. Many people publish many things under the OSR banner, so I'm going to quote from one here, the Principia Apocrypha: Principles of Old School RPGs, or, A New OSR Primer. These are not all the principles, only the ones I will use as examples.
- Rulings Over Rules
- Divest Yourself of their Fate
- Let The Dice Kill Them...
- Leave Preparation Flexible
The first two principles places the "referee" role squarely in the GM's hands. OSR GMs are encouraged by this document to be fair and flexible, neutral arbiters who turn player action and some randomness into life or death for the PCs. The GM is explicitly not the antagonist. Based on this, I feel that the OSR GM has less Adversity than traditional D&D, but only a little less.
The OSR GM has less Authority as well. While not putting all power in the hands of the PCs, the OSR GM is largely reactive to player action and ought to reward player ingenuity, even when it bypasses prior prep.
AW is the father of all PBTA games and codifier of a specific style of gaming. The "Master of Ceremonies" (MC) in this game has specific agendas:
- Make Apocalypse World seem real
- Make the players' characters' lives not boring
- Play to find out what happens
The MC is explicitly told to follow the agendas and principles as rules, and to surrender their own hopes for a scene in favor of "the fiction". In this sense, they get the train rolling out of the station, but where it goes follows rules rather than rulings, and if or when the train goes off the rails, they are right there with the rest of the players. An AW MC has moderate Adversity ("make PC lives not boring", vs. "be a fan of the players' characters") but low Authority. This is not to say that I think the MC has no input in the process, only that they're being told to exercise that input within a set arena.
I believe that Apocalypse World wants to solve the same general problem D&D was solving, but in a different way. The 2E DMG explicitly compares various campaigns to parallel worlds in a multiverse, operating in a way that lets players move from one to another and find themselves in familiar surroundings (rather than landing in a world where the DM has house-ruled the game into unrecognizability). I feel that AW takes this a step further, essentially establishing rules that say "this is what Apocalypse World will be like, and if you want to change these things, you aren't playing AW any more".
Mutants in the Night
There are many indie games I could have chosen to represent the Facilitator, but I picked Mutants in the Night. It is "Forged in the Dark" (based on Blades in the Dark, in the same sense that "PBTA" is based on Apocalypse World).
The BITD SRD talks about the GM role like this:
The GM establishes the dynamic world around the characters. The GM plays all the non-player characters in the world by giving each one a concrete desire and preferred method of action.
Mutants clearly spells out its own philosophy:
The setting provided was inspired by the plight of marginalized people around the world. Mutants hold the key to representing folx of all marginalized backgrounds, and direct representations of real, active and archaic laws that pressure, marginalize, and kill real people every single day.
As such, the goal of my design is to highlight, focus, and empower. The urge to struggle and overcome to break free is a burden and a strength, but compassion, empathy, and community are pivotal to that process.
This is not a game of masters, but of collaborators. Learning is winning. Understanding is success. Create and live in a world to fight for, as you will, undoubtedly, find injustices to fight against.
Emphasis mine. Unlike Blades, the Mutants rules call this role "Facilitator" to make it clear that the game's fiction is a collaborative effort. Like AW, opposition is built into the game, and in this case drawn from the real-world plight of real people. A game like this has a facilitator with very low Authority and Adversity.
High Adversity, Low Authority
GURPS Campaigns (p. 493) suggests that a GM can delegate hostile NPCs to an "adversary" player. Other games, including D&D, support the idea of delegating some of the GM role to another player for similar purposes.
White Wolf's Wraith line put other players in the role of a PC's "Shadow", tempting them or harassing them, and equipped with special rules to do so.
High Authority, Low Adversity
Iyashikei ("healing") RPGs, such as Golden Sky Stories, have no real adversity other than human drama or the challenges of nature to overcome. There is often still a GM or facilitator role, but they exist primarily to portray NPCs and the world, and answer questions about the rules.
In the examples above, it feels like there is a clear line connecting Authority and Adversity - but there are examples of games that break that mold too. I believe that there is a minimum level of Adversity that a typical game can reach before it's not delivering on an RPG's usual goals (iyashikei games aside), but there may not be a minimum level of Authority, as GM-less games do exist. "Where do authority and adversity derive from?" is a question for another post, but I think they don't come from the same place.
Ceding authority to one player has become a subject of much discussion in the indie RPG community. New rules, new games, and other innovations have arisen to challenge the traditional GM role, and some games (and designers) are explicitly GM-less for this reason.