1. The amount of ability, intelligence, or character of a person: Mrs. Smith, the principal of Susan's school, mentioned that they rarely got such a high caliber teacher in theirr school.

Mr. Smart said that they needed to improve the caliber of the schools in the district..

2. The internal diameter of a pipe, cylinder, or the barrel of a firearm: The calibers of a rifle, shotgun, or pistol all have different sizes.
3. The external diameter of a projectile: A caliber can refer to 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.