A raid is a type of mission in a video game in which a number of people attempt to defeat another number of people at a player-vs-player or a number of NPCs in a player-vs-environment battlefield. The term raid itself stems from the military definition of a sudden attack and/or seizure of some objective. This type of objective is most common in MMORPGs, and usually but not necessarily occurs within an Instance dungeon. In RTS games like StarCraft, the term is used differently; see Raid (military).
Raiding originated in the class of text MUDs known as DikuMUD. DikuMUD heavily influenced the game EverQuest which brought the raiding concept into modern 3D MMORPGs. The largest and most popular game to currently feature raiding is World of Warcraft.
The combat encounters comprising a raid usually require players to coordinate with one another while performing specific roles as members of a team. The roles of Tank, Healer, and Damage Dealer are known as the "Holy Trinity" of MMORPG group composition. Other common roles include Buffing, Crowd control, and Pulling (selectively choosing targets with which to initiate combat). A raid leader is often needed to direct the group efficiently, due to the complexities of keeping many players working well together.
Raids are often very rewarding in terms of virtual treasure and items that are unique or that grant exceptional stats and abilities, thus giving players an incentive to participate. Often however, there is not enough treasure to individually reward every player who participates. Players have invented various systems, such as Dragon kill points to distribute loot fairly.