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.
The millimetre (American spelling millimeter) is a unit of length and is a multiple of the SI unit metre. It uses the symbol mm.
There are 1000 mm in 1 m and 10 mm in 1 cm. From this one can derive that there are 0.03937 in.
The millimetre is sometimes verbally abbreviated to 'mil' - which can cause confusion in the US as there is a unit there that means thousandth of an inch.