The term caliber or calibre designates the interior diameter of a tube or the exterior diameter of a wire or rod. It comes from the Italian calibro, itself from q?lib (????), Arabic word for mold.The term most often appears with respect to firearms, as a measure of the inside diameter of the barrel in inches or hundredths of an inch, or in millimetres.
Firearms
In firearms, the caliber is the diameter of the inside of the barrel. In a rifled barrel the distance is measured between opposing lands or grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere.When the barrel diameter is given in inches, the abbreviation "cal" is used in place of "inches". For example, a (smallbore) rifle with a diameter of 0.22 inch is a .22 cal, however the decimal point is generally dropped when spoken, making it "twenty-two calibre".Calibers of weapons can be referred to in metric in millimeters, as in a "cal
Source:
http://en.wikipedia.org/wiki/Calibre