A weasel is a small mammal of the Mustelidae family, which also includes otters, ferrets, and badgers. The word “weasel” originally referred to the European weasel (Mustela nivalis), but it has been extended to refer to other members of the family, such as the American weasel (Mustela vison) or the stoat (Mustela erminea). Weasels vary in length from 15 to 35 cm (6 to 14 in), and usually have a reddish brown upper body, white underparts, and a black-tipped tail.