A measure of drag effects
Often abbreviated BC, ballistic coefficient is a complicated ratio that expresses how easily a particular bullet flies through the air. Ballistic coefficient will (almost always) be a number less than 1, expressed to three decimal places, the larger the number (closer to one), the more efficiently the bullet flies through the air, the lower the number, the less efficiently the bullet flies.
A bullet with a high ballistic coefficient will shoot in a “flatter” trajectory, dropping less over a given distance than a bullet with a lower BC. It will also be less effected by wind drift and will retain more velocity (and, therefore, energy) at longer ranges.