MOA vs Mil
The two angular systems used on rifle scopes: minute of angle in inch-friendly steps, or milliradian in a base-ten metric system.
Both systems measure angle, which is what a scope adjustment actually changes, and the choice between them is mostly about the math you prefer. The minute of angle system divides a degree into sixty parts, with one MOA spanning roughly 1.047 inches at one hundred yards, so it lines up neatly with inch targets and yard distances. The milliradian system splits the circle into 6,283 parts, with one mil covering 3.6 inches at one hundred yards or exactly ten centimeters at one hundred meters.
The practical difference shows up in the turret value of one click: common MOA scopes move in quarter-MOA steps while common mil scopes move in tenth-mil steps. Neither is more accurate, but a shooter should match reticle units to turret units and stick to one language so holds and dials always speak the same dialect.