“Similar to the common shell except that the walls of the projectile are thinner. In both spherical and rifled projectiles, the bursting charge was usually located in a thin metal container, commonly made out of tin or iron. The case-shot material was filled around this container. The internal cavity was usually filled with lead or iron balls in a sulphur pitch matrix. The small bursting charge of black powder was designed to disperse the case-shot balls in a cone-shaped pattern. The concept was to give the same effects of canister but at much longer ranges. Case-shot was invented by an Englishman named Lt. Henry Shrapnel of the Royal artillery in 1784.
The above definition is from Melton & Pawl’s “Guide to Civil War Artillery Projectiles", by Jack W. Melton and Lawrence E. Pawl, in the Glossary found on page 88.