The ferret is a domestic mammal of the type Mustela putorius furo. Ferrets are exually dimorphic predators with males being substantially larger than females. They typically have brown, black, white, or mixed fur, have an average length of approximately 20 inches (51 cm) including a 5 inch (13 cm) tail, weigh about 1.5–4 pounds (0.7–2 kg), and have a natural lifespan of 7 to 10 years.
Several other small, elongated carnivorous mammals belonging to the family Mustelidae (weasels) also have the word "ferret" in their common names, including an endangered species, the Black-footed Ferret. The ferret is a very close relative of the polecat, but it is as yet unclear whether it is a domesticated form of the European Polecat, the Steppe Polecat, or some hybrid of the two.
The history of the ferret's domestication is uncertain, like that of most other domestic animals. It is very likely that ferrets have been domesticated for at least 2,500 years. They are still used for hunting rabbits in some parts of the world today, but increasingly they are being kept simply as pets.
Being so closely related to polecats, ferrets are quite easily able to hybridize with them, and this has occasionally resulted in feral colonies of ferret polecat hybrids that have been perceived to have caused damage to native fauna, perhaps most notably in New Zealand. As a result, some parts of the world have imposed restrictions on the keeping of ferrets.