7.63x25mm Mauser, or simply 7.62 Mauser, is a rimless, bottleneck pistol catridge designed in Germany by Deutsche Waffen- und Munitionsfabriken, and was the original cartridge used by the Mauser C96 pistol and made its way into other well-known German pistols.
The 7.63x25mm Mauser was designed in 1896 by Deutsche Waffen- und Munitionsfabriken (DWM), based on the 7.65x25mm Borchardt cartridge. It was the original cartridge used by the Mauser C96, which would later use different rounds later on. It wasn't just used in the C96, which shouldn't be a surprise considering that most cartridges are used in multiple firearms. Another notable use for the round is the Astra Model 900, a Spanish copy of the C96.