A mil is short hand for milliradian. Milliradian defined is as the angle formed when the length of a circular arc equals the radius of the circle.
A milliradian, is the angle formed when the length of a circular arc equals 1/1000 of the radius of the circle.
Summerized it is an angular measurement. Similar to MOA in that it is also a angular measurement, but different incremental value.
A common misconception is that MOA adjustment (in traditional riflescope) and Mil are linear measurements. They are not!
MOA (minute of angle) is an angular measurement and so is (MIL = short for) miliradian. It is a coincidence that when MOA is converted into a linear measurement that it subtends 1.047 inches at 100 yards. That is so close to 1” that most people round down and consider it to be 1” at 100 yards. So a scope with .25 MOA adjustments translates to each click moving the reticle .25 of an inch at 100 yards.
However, the fact that 0.1 mrad is 1cm at 100 meters is most certainly not a coincidence. That is most certainly intended that way by design and comes out of the very definition of the angular measurement of a radian and of the method behind the metric system.
That is the beauty of the metric system: you can start with very few basic unitless measurements and scale everything from there.
1 radian is simply an angle where the subtended arc is equal to the radius, for example 1mrad is 0.001 of a radian, so 1mrad is an angle where the subtended arc is one thousandth of the radius, for example if an object at a certain distance from you subtends exactly 1 mrad, then the size of that object in linear units is almost exactly one thousandth of a distance between you and that object.
What linear units you happen to use makes no difference. It will work with either of them.
It is simplest to use with metric linear units because of the base10 scaling, of course.
With moderately retarded measurement systems we use for traditional reasons (like inches/feet/yards for linear units and degrees for angular units)you know for common things like to measure a trailer or perhaps building something, translation between angular and linear measurements is a lot more involved.
Hence, between MOA and MRAD, for as long as all you need to do is trajectory compensation, it does not matter what you use.