Acronym for "Global Reservations System". These used to be known as CRSes, back when airlines owned them and their primary purpose was to do airline reservations. Nowadays, GRSes handle car rentals, hotel rentals, tours, and cruise reservations, in addition to airline reservations, although air travel still provides the bulk of the traffic. There are 4 major GRS companies: Sabre (still partly owned by American Airlines), Galileo (partly owned by United, marketed as Apollo in the USA), Worldspan (a partnership between Delta, Northwest, and TWA, which has just been bought by American...this gets complicated), and Amadeus (owned by Lufthansa and I believe some other airlines). As far as I know, all 4 still run on IBM Mainframes using TPF, though there have been efforts by these companies and others to build large scale reservations systems on other platforms.