A rulebook or rule book, sometimes also called a core rulebook, is a publication that contains the rules and setting information that are necessary to play a role-playing game. When the rules of a game are published in a format other than a book like a zine or a single page, they may or may not be called a rulebook.
A given role-playing game may have one or more rulebooks, but most rulebooks are specific to a single game. For example, the Fate Core rulebook contains everything needed to run the game, and any other publications are supplements to that rulebook even if they contain new rules. However, many editions of Dungeons & Dragons split their core rules into three rulebooks (the Player's Handbook, Dungeon Master's Guide, and Monster Manual) that are all needed in order to run the game.