Caliber refers to the internal diameter or bore of a firearm's barrel, typically measured in millimeters or inches. It is an important specification that affects the type of ammunition a firearm can use, influencing factors such as accuracy, range, and stopping power. For example, a 9mm handgun has a barrel diameter of 9 millimeters, while a .45 ACP pistol has a barrel diameter of .45 inches. In addition to firearms, the term caliber can also describe the quality or degree of excellence of a person or thing. For instance, one might refer to a caliber of a professional athlete or a caliber of a literary work, indicating their level of skill or significance in their respective fields.