Sectional Density
A bullet's weight in pounds divided by the square of its diameter in inches. Higher sectional density means more mass behind the bullet's frontal area.
A 175 grain .308 bullet has higher sectional density than a 110 grain .308 of the same diameter. The denser the bullet relative to its cross-section, the better it cuts through air and the harder it hits.
Sectional density is one of two ingredients in ballistic coefficient, and the other is shape. A long, heavy, well-shaped bullet has both.