Gamemaster's screen
A gamemaster's (GM) screen, also called dungeon master's (DM) screen, is a gaming accessory, usually made out of cardboard, used by the gamemaster to hide all the relevant data related to a tabletop role-playing game session, hiding them from the players in order to not spoil the plot of the story.
General description
Very often the front side of the screen just displays an illustration, usually related to the game's subject. The reverse side is most of the times filled with tables, rules and other useful gaming data.
Some role-playing games have an adapted name for their gamemaster's screen. Dungeon Master's Screen is the term used, for example, for the Dungeons & Dragons gamemaster's screen.
In addition of the official gamemaster's screens sold by professional gaming publishers, some customizable models are also available.
Examples
Elric!: Gamemaster Screen
Star Wars: Gamemaster Screen for Second Edition
Hollow Earth Expedition Gamemaster Screen
Pathfinder Roleplaying Game: GM's Screen