A very long, very detailed description of every aspect of a game/simulation. It includes screen mock-ups, user-interaces, basic game-play, art and animation assets, units, maps, state-charts, character descriptions and artwork, story-boards, and high level equations and relationships. It is not a marketing document, but may include a summary of the goals of the game/sim. The design document for an Xbox game, for example, is around five hundred pages. It can also be called the functional specifications. There is also a technical design document that explains how items in the design document will be implemented.