D20

d20, n. abbrev., p.n. 1. abbreviation for a twenty-sided die. 2. The game system used in Dungeons & Dragons, 3rd Edition and 3.5. 3. a universal RPG system, originally based on the D&D 3rd Edition rules, that allows for adaptation and use in differing genres and settings.
----
Addendum #1: Seen by many as the savior of gaming (and others the bane), the d20 System spread like wildfire across the RPG industry in the '00s, as many companies (examples include Green Ronin and Mongoose Publishing) were created initially to produce d20 books, settings, and modules. A number of original settings and systems were also printed versions in the d20 system, including Deadlands, Star Wars, Traveller, Call of Cthulhu, and Legend of the Five Rings. With the shift of D&D to 4th Edition, the OGL was still kept, but the number of companies printing for it dropped.
Addendum #2: A number of reasons for the d20 System's popularity include:
1. Freedom to create a setting rather than focusing on creating a brand new system to go with it. (This reason is often cited by critics of d20 as a stifling of creativity of new systems.)
2. Since many gamers get their start playing D&D, they are already familiar with the rules, since most d20 games don't deviate from the main system too much, and can work quickly on making a character rather than learning another set of rules. (Often a d20 fan's rebuttal to the first argument)
3. Ability to bring classes, characters, or rules from other d20 products into a game with relative ease, instead of figuring reams and reams of conversion tables.
4. As mentioned earlier, a number of already existing systems and settings had versions written for the d20 System.
5. The large number of d20 products available. (Of variable quality, of course.)