The caliber is a unit of length based on the inch (imperial units) and uses the symbol cl.
It represents the internal diameter of a gun barrel and therefore more often than not, the size of the ammunition or round that gun requires to fire.
It is not always represented or converted exactly in inches so a table or "well-known" conversions are often used to depict measurements / ammo types instead.
The mil is a unit of length in the imperial unit system and uses the symbol mil (or sometimes thou).
A mil in one one-thousandth of an inch (0.001 inches) or 25.4 μm.
The mil is still used in the US to measure thickness of various materials.