Standard Deviation
Also: SD
A statistical measure of how tightly your shot velocities cluster around the mean. Lower SD predicts smaller vertical dispersion at distance.
SD is the bullet-flight version of the bell curve. About two-thirds of your shots will fall within one SD of the mean velocity; about 95% within two. Lower SD means a tighter cluster.
A precision load aims for SD in single digits. At 1,000 yards, even a 10 fps spread can move impact several inches vertically, and at 1,500 it can stretch to roughly six feet of vertical error.