From The Free Online Dictionary:
mil (ml)
A unit of length in the US Customary System equal to 1/1000 of an inch (0.03 millimeter), used chiefly to measure the diameter of wires.

This measurement, also used to measure thickness of plastic, has nothing to do with the metric system!
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey