The term "generic" has been used since the earliest days of gaming to describe a system that can be used for any type or style of game. The Fuzion 5.02 rules uses the term to describe its basic ruleset as separate from its Champions and Interlock forerunners. In the second paragraph of the introduction to GURPS 3rd Edition the authors define "generic" as a means to satisfy players and game masters of many styles of play and feel for rules. This is repeated in the updated 4th edition rules along with acknowledgments to Champions as the first truly flexible character creation system.
It is frequently disputed whether d20, which requires massive alteration with each new genre because of its class and level system, really qualifies as a generic system. Some d20 derivatives, however, such as Green Ronin Publishing's True20 Adventure Roleplaying are presented as fully generic systems.
Other influential generic systems include:
Many role-playing games have rules designed for a specific genre such as sword and sorcery, or specific universes such as Star Wars. Conversely, a generic system has basic rules designed to handle the wide variety of situations that can arise in the spectrum of settings. There is a long-standing dispute among RPG professionals over whether any system can be truly generic and whether it produces better games to use them or to build the system around the desired setting and genre.
The advantage of a generic rule system is that players only need to buy and learn from one main rule book, saving money and time. Since most settings share a large set of features, such as the way characters move and fight, players would not have to re-learn such basics when starting in a new setting using those generic rules. This eases players in the move from one setting to another. The players and the game master choose the desired setting before starting a game.
Generic game systems also have their share of disadvantages. Often, the basic rules of a generic system are more complex than those designed for a specific setting. Generic rule books have to cover features and aspects that might be of little use in some settings. Also, game developers still might need to produce rule books designed for specific settings to provide meaningful gameplay. If the developers take this too far, it can offset the original advantages of a generic system in the first place.
A cross-genre or multi-genre role-playing game allows the exploration of different genres in their own settings with the same character, or different versions or incarnations of the same character. The character may go from the American Old West, to four-color superheroics, to a mystery, and back to an invasion of Earth by aliens. While fantasy and science fiction are usually the most common genres, others can include mystery, western, horror, historical, and espionage.
This doesn't strictly require generic rules; even first edition Advanced Dungeons & Dragons, clearly not a generic system, had rules for interacting with Boot Hill and Gamma World settings and characters. However, generic rules clearly make this crossover easier.
Taking a character from one world to another can be one of the strengths of a generic system and may be the point of the game. The default GURPS 4th Edition setting, for example, takes advantage of exactly that strength. However some otherwise generic games do not always allow for transferring the same character to different worlds with the same rules. The d20 system, for example, has special setting-specific rules that mean that you cannot take a character out of one world and put him in another without adding rules to the new world to accommodate him. FUDGE, because of the looseness of the rules system, GURPS and the Hero System, through design, do not need to have rules added for a world change - although a little character adjustment may be necessary because of the different way the new world sees the character (e.g., in a new world the fact that a character is an officer of the law in the previous world may no longer be relevant except for character background).
On the other hand, a multi-genre or cross-genre role-playing game does not have to be the same as a generic game system, since it does not have to have rules for all possible genres, just those for the specific settings or worlds it covers. The below are examples of multi-genre, but not completely generic, role-playing games.