A shell was a type of projectile that usually contained an explosive, incendiary, or other chemical payload. Shells were launched from artillery weapons such as mortars, cannons, and tank guns.
In 2376, The Doctor recounted that while he was living in the Central Protectorate on Kelemane's planet, there was a war that started when the neighboring state decided to lob a few cannon shells at them. (VOY: "Blink of an Eye")