MOA refers to "minute of angle"...basically it is a measurement of the angle of dispersion of rounds at whatever range you are shooting...

Angles are measured in degrees--minutes--seconds. One (1 MOA) minute of angle is 1/60th of a degree. This is just over 1 inch (1.047...) at 100 yards...2 inches (2.094...) at 200 yards, etc.

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Similarly, mil-dot scopes measure distance based on the milli-radian or 1/1000 radian. 1 "mil" equals 3.6 inches at 100 yds. The catch here is that most scopes only accurately measure this on the highest magnification.