The 9 millimeter Luger and 9 millimeter Parabellum ammunition are the same cartridge with different names. They are both the 9x19mm ammunition cartridge, commonly referred to as simply 9x19.
The 9x19 Parabellum cartridge was designed by Georg Luger in 1902. The name Parabellum comes from the latin phrase "Si Vis Pacem, param bellum," which means "If you seek peace, prepare for war." The 9x19 was originally intended for the German Luger semi-automatic pistol. Because of this, 9x19 is also referred to as 9 mm Luger. While other variants of the 9 millimeter cartridge exist, including the 9 mm Makarov and 9 mm Browning, both of which have unique dimensions differentiating them from the 9 millimeter Parabellum, the 9x19 remains the most popular variant of 9 mm ammunition today. The 9x19 is also the only authorized variant of 9 millimeter ammunition for North Atlantic Treaty Organization (NATO) countries. For this reason, it is also referred to as 9 mm NATO. The dimensions of the 9x19 millimeter Parabellum cartridge are a bullet diameter of 9 mm, case length of 19.2 mm and cartridge length of 29.5 mm.
Ammunition stamped as 9 mm Parabellum and 9 mm Luger will both work interchangeably in any weapon chambered for 9x19 mm ammunition. It is important to remember that other variants of 9mm ammunition exist that will not function properly in a weapon chambered for 9x19 mm.