Yamaha XG is an extension to the General MIDI standard created by Yamaha. The XG stands for "extended general". It was created to compete with Roland's GS standard.
Yamaha also released XG-enabled computer sound cards, starting with the SW60XG ISA soundcard and DB50XG wavetable daughterboard.
Yamaha eventually discontinued its XG software products, stopped manufacturing XG sound cards, and stopped including XG support in its synthesizers. With the creation of the General MIDI 2 standard, there ceased to be a need for the XG extensions.
Partial list of XG synthesizer hardware:
XG support could also be added to many of Yamaha's synthesizers by installing a PLG100-XG expansion board.