The nanometre is a unit of length and is a multiple of the SI unit metre. It uses the symbol nm.
1 nm is equal to one one-billionth of a metre. (Nano is the prefix for one billionth). It is used to express dimensions on an atomic scale for example a helium atom has a radius of approximately 0.03nm.
The nanometre was previously known as the millimicrometre (or the millimicron).
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.