Polecat

Polecat is the common name for several medium-sized Mustelids. They do not form a single clade: the name is given to several similar species. 
The European polecat is typical, and is the only one native to the British Isles. Ferrets are the domesticated form of European polecats.
In the U.S., the term "polecat" is often applied to skunks, which are only distantly related to the European polecat. The reason is that polecats give off a scent which some people think is offensive. 
Taxonomy.
The simplest version puts them all in a subfamily of the Mustelidae:
Subfamily Mustelinae
There is a more recent version which splits them up even more.