The Minute of Angle is a measurement that is typically used to assess the accuracy of a firearm. It is usually summarized with the correlation of one minute of angle to a perfectly focused firearm missing its target by a single inch at the distance of 100 yards.
Origin of the Term:
In the traditional measurement of an angle, a minute is 1/21,600 of a full rotation around a stationary point. It follows that the term "minute of angle" refers to the number of minutes that describe the arc that a projectile travels as it approaches its intended target.
Why Does It Exist?
I know that there are many physics experts on E2, and that this terribly elementary, but I will elaborate on just why a projectile has minute of angle. When an object is fired in a gravitational field, it will pull that projectile from its course and towards the origin of the field. It doesn't matter how fast the projectile is traveling -- this gravitation is inevitable. However, the effects of gravity compound over time, so the faster the projectile gets to its target, the less gravity will redirect it, and the less minute of angle it will have.
Every projectile has a minute of angle. The measurement tends to be applied to firearms, which are precision tools, but can also be used in an analogous manner to a bow launching an arrow, or a sling launching a calculus. The matter of minute of angle is mathematical, not subjective; it is in no way limited by what it is describing.
In this vein, it should be noted that minute of angle is actually the measurement of any directional aberration in a projectile's course, and that I highlighted the affect of gravity on it because in the case of most of its applications, minute of angle is used only to gauge the slope of a bullet's travel as results from gravitation.