a. The diameter of the inside of a round cylinder, such as a tube.
b. The diameter of the bore of a firearm, usually expressed either in millimeters, in inches, or as a decimal fraction of an inch: .45 caliber.
c. The diameter of a projectile such as a bullet or artillery shell, corresponding to the caliber of the firearm from which it is to be fired.
2. Degree of worth; quality: a school of high caliber; an executive of low caliber.
[French calibre, from Italian calibro, from Arabic qālib, qālab, mold, shoe tree, from Greek kālapous, shoemaker's last : kālon, wood + pous, foot; see ped- in the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.