Definition: A co-op game is a game in which two or more players team up to achieve a specific goal, playing side by side, either via LAN, split-screen, or via the Internet. More specifically, co-op is a multiplayer game play type. The literal translation is co-operative. Co-op games are widely popular and are increasingly becoming an included game play option in many games.
Also Known As: coop
Examples:
There are many co-op games available on most major systems, but my personal favorites are the Terrorist Hunt and Mission modes found in Rainbow Six 3, by Ubisoft.