In technical discourse, however, as in American usage, the term 'weasel' can refer to any member of the genus, or to the genus as a whole. Early literary references to weasels, for example their common appearances in fables, refer to this species rather than to the genus as a whole, reflecting what is still the common usage in the United Kingdom. Originally, the name 'weasel' was applied to one species of the genus, the European form of the Least Weasel (Mustela nivalis).
Weasels are mammals in the genus Mustela of the Mustelidae family.