shell
A general term for the calcareous exoskeleton that protects the soft insides of many animals. Humans use them for a wide variety of purposes.
Most commonly seen as but not limited to seashell-dwelling mollusks on Danbooru. If you aren't tagging a mollusk, it's probably also turtle shell.
For the shells of eggs see eggshell. For cannon projectiles, see artillery shell. For spent ammunition, see shell casing.