The word "MMO" has been redefined countless times since the early days of MUDs and social games. It’s a contentious term with personalized definitions, and as the market has grown so has the scope with which the label is applied. Narrowing down a definition of "massively multiplayer online game" that is still broad enough to encompass the growing range of titles cannot rest on one concrete definition. But in the interest of clearly labeling games, the following features are highlighted as possible indicators of an MMO. Not every MMO will embody each of the listed marks, but it will feature some of them. And depending on how many and how central they are to gameplay a title is considered to be an MMO.

So What Defines an MMO?

Persistent or Instanced Based World

MMOs typically have persistent or instance-based worlds, allowing players to interact in real-time. Ultima Online and EverQuest are popular examples of early persistent world MMOs. Their environments are linked together so that players seamlessly travel from one area to the next. Or more recently, in World of Warcraft players can run from the Ruins of Silvermoon to Booty Bay without loading each zone—but I don’t recommend the journey.

Other MMO’s employ instanced worlds. Players join a central hub and then branch out to zones. Each area is not connected but exists independently from the rest, such as in The Secret World, so that players must load each zone. Although, many open world games also rely on instanced zones for dungeons, PvP battlegrounds, and specialized areas for other events. So that, many MMOs end up with a melange of both persistent and instanced worlds.

Common Characteristics

Along with shared world building traits the MMO has evolved common characteristics. Oftentimes an MMO has a virtual economy, where players trade in-game goods. There are various degrees of markets depending on the particular game. Eve Online has a robust player-driven economy, where some players manipulate the market for their own financial gain. Whereas other games rely on simpler auction houses. But not every game has a market either. Diablo 3 disassembled their auction house, and in its current state there is no form of trade, even directly between players.

Character customization has also become a hallmark of MMOs—the option to create a distinct avatar; whether it be a ridiculous mishmash of human anatomy in a game such as Perfect World, or the polished realism of Black Desert. But avatars are not restricted to personifications of people.

MOBAs and TCGs

Games like League of Legends and Heroes of the Storm are part of unique genres but they are often grouped under the umbrella of MMOs. They don’t offer instanced dungeons or persistent worlds, but they embody characteristics typical to the genre. Players in trading card games often personify a character, whether it be themselves immersed in the world of the card game or by taking on the role of an avatar inseparable from the game’s world, i.e. Jaina Proudmoore in Hearthstone. And customizing a deck is akin to designing your character. Instead of stats and character traits you design a deck that distinguishes your play style from other players, and your deck evolves as you unlock new cards and become further acquainted with the game’s mechanics.

Many MOBA’s also offer customizable characters through items, skins, and abilities. As well as a leveling system that serves as a sign of one’s skill. But they also feature microtransactions, a typical trait of free-to-play MMO titles, although not exclusive.

About “Massive”

A typical rebuttal is that 5v5 matches are too small for a MOBA to be considered an MMO, not deserving of the term “massive.” But in the context of MOBAs massive isn’t used to describe an individual game between players, but the shared server in which everyone plays, or the environment in which players are grouped together. And with 540,000 concurrent players on average each month, Dota 2 seems to qualify as "massive."

It is difficult to narrow down a strict rule when determining how many players constitute massive. Ten players may not be enough to embody the term but what about 32? Where is the cutoff? If I start with one grain of sand and keep adding one grain every second, at what point does it become a pile? The same issue exists with the terminology. A satisfactory definition of "massive" cannot be exclusively based on the number of players. If players can play a game where each instance of play is potentially grouped with new players, then enough players exist for the game to be considered massive.

MMO Features But Not An MMO

Many games are beginning to feature MMO characteristics, but they’re not considered MMOs. Grand Theft Auto V’s online play includes microtransactions, levels, and a massive world, but it is not an MMO. The core experience of an MMO is online play, but the online features in games like GTAV are secondary attractions, not the focus of the game’s narrative. Also, the interactions between players are not the driving force behind the online experience of Grand Theft Auto V. It is possible to play the online portion of GTAV without working with other players. Whereas, in order to achieve a win-state in Final Fantasy XIV (in terms of item collection) players must group and coordinate together. And player interaction is a core element of any MMO.

Buy-to-play VS. Free-to-Play

Surveying the landscape of MMOs it's apparent there are two camps. Free-to-play MMOs, abbreviated F2P, offer players content without the need to spend real money on the game. But they feature premium content, that comes in a variety of flavors, and varies from game to game. Numerous F2P titles offer cosmetic items, whereas others offer powerful items that give players an advantage over fellow players. Regardless, premium content must be acquired with an in-game virtual currency, purchased with cash. Or, players may be able to purchase items with currency earned in-game.

On the other hand, pay-to-play games are either purchased for a flat cost, such as The Elder Scrolls Online, or require a monthly subscription to continue playing, such as Dark Age of Camelot. Although the latter seems to be a dying breed, as more developers transition to a flat cost or free-to-play model.

Many new titles are hybrids of both genres, whereby a pay-to-play game may offer premium content such as cosmetics, or a free-to-play title has the option to purchase a monthly subscription for additional perks, such as Skyforge. But free-to-play only refers to games that allow players access to all core content without paying. Games that offer free-to-play elements but require paying to proceed through later game content, like Wizard101, are not truly free-to-play titles. But keep in mind that essential content does not refer to items such as experience boosts or cosmetics.

MMO is still an evolving term that is sure to include new features in the future. Many survival games have are now grouped as MMOs, by offering persistent worlds, microtransactions, a massive amount of players, and a core experience revolving around online play (i.e. Rust). As with any word, the term will change, but the previously listed features are guiding themes to be used when dissecting a particular game.

If I missed any features you think define an MMO, leave a comment below.