The word "caliber" is often used to describe the degree of merit or excellence, according to Dictionary.com. Caliber is also a unit of measurement for the size of a bullet.
Calibers can be expressed either in inches or millimeters. For example, a .357 caliber bullet has a diameter of .357 inches and a 9 mm caliber bullet measures 9 millimeters.
The caliber of the bullet is determined by measuring the slug portion of the bullet cartridge. This is an important distinction because in some cases the bullet casing is wider than the slug itself. It is also important to note that in spoken language the decimal point is usually dropped. For example, a .44 caliber bullet is said to be a "44 caliber bullet."