The protective shell covering a turtle’s back is called the carapace.
A turtle shell has two principal parts: the domed carapace on the back and the plastron on the underside. These structures are connected by bridges and are formed from bone covered by keratinous scutes in most turtles.
The shell is part of the skeleton rather than a detachable armor plate. Turtle ribs and vertebrae are incorporated into it, which is why turtles cannot simply leave their shells. The shell protects vital organs and also provides attachment surfaces for muscles.
People often use “shell” for the whole structure, but carapace specifically means the upper portion. “Plastron” means the lower portion, while “scute” refers to an individual keratin plate covering the shell’s surface.