1. The amount of ability, intelligence, or character of a person: "We rarely get such a high caliber teacher in our school."

"We need to improve the caliber of our schools."

2. The internal diameter of a pipe, cylinder, or the barrel of a firearm; such as, a rifle, shotgun, or pistol.
3. The external diameter of a projectile; such as, a bullet or a shell which is fired from a particular weapon.
4. Etymology: "degree of merit or importance", is from Middle French calibre in the late 15th century; apparently it ultimately came from Arabic qalib, "a mold for casting".

Arabic also used the word in the sense of "mold for casting bullets"; which is the original literal meaning in English.