A terrapin is a turtle that lives in brackish water. They are named for the diamond pattern on their shells. They eat mollusks, crabs, and fish. Terrapins are sexually dimorphic. Females are much larger than males. Males range between 4 and 5 inches. Females average nearly 7 inches. The largest terrapin on record was a female measuring 9 inches. A terrapin is one of several small species of turtle living in fresh or brackish water. Terrapins do not form a taxonomic unit, and may not be related. Many belong to the families Geoemydidae and Emydidae.
The name "terrapin" is derived from torope, a word in the Native American Algonquian language that refers to the species, Malaclemys terrapin. It appears that it became part of common usage during the colonial era of North America and was carried back to Great Britain. Since then, it has been used in common names for turtles in the English language. In the UK, red-eared sliders are known as red-eared terrapins.