There are currently 3 names in this directory beginning with the letter S.
Sectional density (SD)
When referring to a bullet, the Sectional density (SD) refers to the weight of the projectile relative to its caliber. SD is most often used when comparing the penetrating ability of a projectile in one caliber to another.
Second Focal Plane. Referring to a rifle scope which has the reticle on the second focal plane so the reticle is not changed when increasing or decreasing magnification as it does not move.
Standard Deviation (SD)
The measure of how spread out your velocity between loads are from the baseline. Difficult to estimate so ES (extreme spread) is often simpler to use.