Minute of Angle. It's a way of measuring accuracy. There are 360 degrees in a circle and 60 minutes in a degree. So if you start a cone at the rifle's muzzle and project it out to the target it gets progressively larger the further out you go. One minute is roughly a half inch at 50 yards, one inch at 100 yards , 2 inches at 200, etc. It allows you to consistently describe accuracy regardless of distance to target.