Caliber is a unit of measurement related to a bullet’s size. Specifically, what’s being described is the bullet’s diameter. To be even more specific, the caliber of a bullet is the measurement of the diameter of the slug (or projectile) part of the bullet cartridge. This is important because in some situations, the bullet casing will be wider than the slug itself. But the caliber is specifically describing the width of the slug portion. Caliber does not refer to the length or power of the bullet (in most cases), but simply the diameter of the bullet.