One-pager
Also said as One-page design, Pitch sheet
A single page that says what a game or a system is: what the player does, who it is for and why it is exciting. If it won't fit on one page, you probably haven't decided yet.
People read one page. They will also pin it to a wall and use it. It forces you to choose what matters.
Say it
“Write me a one-pager on the fishing system before we schedule it: what the player does, one drawing of the screen, and the one reason it makes the game better.”
Where you’ve seen it
Stone Librande, a designer on Spore and Diablo III, gave a talk at GDC in 2010 on one-page designs: single illustrated sheets, drawn like posters and pinned up for a team, as a replacement for long documents.
Not to be confused with game design document
A one-pager is short enough that everybody reads it. A design document is long enough to answer questions, so people only look things up in it.