A "WC" in a bathroom is a water closet, which refers to a flush toilet. The term "water closet" generally means that the toilet is confined to a very small, enclosed area.
A "WC," or water closet, is not to be confused with a bathroom, which also contains a tub, shower or other amenities. Water closets contain only the flush toilet and possibly a sink or basin. They are sometimes separated from the rest of the bathroom by a door to provide privacy, contain odors or for aesthetic purposes. In Europe it is not uncommon to see "WC" signs signifying the availability of small, public flush toilet rooms.