Minute of angle. As it sounds, its an angular measurement. Like a circle divided into 360 degrees, each degree is divided into 60 minutes. The further out from the the radius of the circle, the greater the distance covered by a minute of angle. At 100 yards, it covers approximately 1". At twice the distance, one moa covers twice as much, so 2" at 200 yards. Scopes are typically adjusted by measurements of 1/4 moa (sometimes 1/8, 1/2 or even 1 moa). At 50 yards, 1 moa is 0.5". When a guy says his gun shoots 1 moa, that usually means out of all the times he's shot his rifle, once he fired two shots that were only 1" apart at 100 yards.