Core is a term that is used in a variety of contexts to mean something essential, central, and necessary. It can be used in a number of ways.
- A core rulebook is one of the rulebooks that contains what are considered the necessary rules to play a standard game. For example, in GURPS 4e, the two books, Characters and Campaigns, cover everything necessary to run a standard GURPS game.
- A core mechanic is a general method of resolution that is used in multiple domains and contexts. For instance, in d20, the core mechanics of rolling a d20 and adding a modifier, which is then compared to a target difficulty (DC), is used both in combat and in skill checks (different domains), and in both Swim checks and Sense Motive checks (different contexts).
- A core library includes the core rulebooks as well as any supplements which are assumed to be used in a standard game. For instance, in GURPS 3e, two Rules Compendiums were considered part of the core library, including canonical versions of various traits and rules as they would be used in future GURPS sourcebooks and other official sources.