A tortoise is a land reptile, similar to a turtle, which is protected by a hard shell on its back.
Dudley once threw his pet tortoise through the greenhouse roof (PS3).
The gigantic tortoise with jewel-encrusted shell that Harry, Ron, and Hermione saw in the Magical Menagerie was probably a Fire Crab (PA4).
During their exams at the end of their third year Harry and his classmates were each asked to transfigure a teapot into a tortoise. A number of students' tortoises still had teapot-like traits, and Hermione worried that hers bore more resemblance to a turtle than a tortoise (PA16).