Game design document
Also said as GDD, Design doc
The written description of what a game is and how it works: its rules, systems, content and goals. On a good team it is a living reference that gets changed, not a contract.
Everyone on a team needs one place to check what was decided. Written too early and too long, though, it describes a game nobody has played yet, and nobody reads it.
Say it
“Keep the design doc to how each system works and why, with one page per system. Update it the same day a playtest changes something, or delete that page.”
Where you’ve seen it
The Doom Bible, written by Tom Hall at id Software in 1992, planned a story and characters in detail. Most of it was thrown away as the game became what John Carmack and the team found was fun to play.
Not to be confused with one-pager
A design document holds the detail of how things work. A one-pager sells or summarises the idea, and fits on a single sheet.