Milliradians, a division of radians, it's a standard angle measurement. Used in a lot of fields, not just lasers. Especially if you're talking about deviations less than a degree - specifically small divergences over long distances. (Satellites/microwave, astronomy, lasers of course, and in some cases firearms.) If you have a circle of radius x, one radian is the angle that occurs when the outer arc length is equal to the radius of that circle.
One milliradian would be 1/1000th of that. If your laser source point (diode or OC mirror) is the 'center of the circle', and x measurement along your beam is the radius, the width of the beam at x feet would be your arc length.
The angle developed between source point, along beam of specified (x) distance when the beam is at thickness gives you your mRad reading.
so radians=n divided by x; or radians = width of beam at distance x, divided by x. (same units.) With a laser that number will be far <1, so you'd measure it in milliradian or millirads.
EDIT: Guess it was a troll, but, leaving it here for future searches.