Mil
Also: Milliradian · MRAD
An angular unit equal to 1/1000th of a radian, about 3.6 inches at 100 yards, or one yard at 1,000 yards. The standard unit on most precision optics.
A milliradian is the unit most modern precision optics use, including nearly everything from Europe and most US tactical glass. The math is elegant: at any distance, one mil equals one-thousandth of that distance. At 1,000 yards, one mil is one yard. At 600, it’s 21.6 inches.
A scope with 0.1-mil clicks adjusts about 3.6 inches per click at 100 yards. Slightly coarser than 1/4-MOA inside 200, slightly finer past 600.