A cartridge for a shotgun.
Synonym: shotgun shell
A shotshell is assembled from a hull, a primer, propellant, a wad, and projectiles. A shotshell normally contains multiple projectiles, called “shot”.
Colloquially, only the most pedantic users use the word “shotshell”, and instead normally use the term “shotgun shell” or simply “shell”.
Although a shotshell is technically a cartridge, a shotshell is almost never referred to as a cartridge.