A unit of measure that is 1/1000th of an inch. The term can also be used interchangeably with "thou" witch is simply much shorter than saying "thousandths of an inch". The advent of the metric system has generated some confusion as some have used the term "mil" as an abbreviation for the millimeter (mm). The only thing that they share is that each is 1000th of something. The traditional "mil" is 1 thousandth of an inch and the millimeter is 1 thousandth of a meter.
Although the unit seems impossibly small it is very common in industries where an inch is absurdly large such as film (for plastic bags), foil, and wires.
1 mil = 1 thou = 0.001" = 0.0254mm
This is a pretty precise measuring tool, it says that a single strand of my hair is 2 mil thick.
No wonder the bags keep ripping there only 1 mil thick. See if you can order 3 mil bags.