A Massively Multiplayer Online Game (also called MMOG, MMO) is a computer game which is capable of supporting hundreds or thousands of players simultaneously. By necessity, they are played on the Internet, and feature at least one persistent world. Some argue that small player-count games, with 200 and fewer players, are also part of the genre; the persistent world is probably the only "hard" requirement.

MMOGs can enable players to cooperate and compete with each other on a grand scale, and sometimes to interact meaningfully with people around the world. They include a variety of gameplay types, representing many video game genres. Many MMOGs require players to invest large amounts of their time into the game. Most MMOGs require a monthly subscription fee, but some can be played for free.