A game design document (GDD) is a comprehensive blueprint for a video game. It outlines the game's concept, mechanics, story, characters, and art style. The GDD serves as a reference for the development team, ensuring everyone is aligned on the vision and goals of the project. Typically, a GDD includes sections on gameplay mechanics, level design, user interface, and sound design. It may also detail the target audience and platform, such as PC, console, or mobile. A well-structured GDD helps streamline the development process and can be crucial for securing funding or support from stakeholders.