Tortoise and Turtle

Tortoise noun - Usually herbivorous land turtles having clawed elephant-like limbs; worldwide in arid area except Australia and Antarctica.

Turtle is a synonym for tortoise in tortoiseshell topic. In some cases you can use "Turtle" instead a noun "Tortoise".

Turtle noun - Any of various aquatic and land reptiles having a bony shell and flipper-like limbs for swimming.

Tortoise is a synonym for turtle in reptile topic. You can use "Tortoise" instead a noun "Turtle", if it concerns topics such as tortoiseshell.