A Butterfly knife, also known as a fan knife or Balisong, is a folding pocket knife

Pocket knife

A pocket knife is a folding knife with one or more blades that fit inside the handle that can still fit in a pocket. It is also known as a jackknife or jack-knife...

 with two handles counter-rotating around the tang

Tang (weaponry)

A tang or shank is the back portion of a tool where it extends into stock material or is connected to a handle as on a knife, sword, spear, arrowhead, chisel, screwdriver, etc...

 such that, when closed, the blade is concealed within grooves in the handles. It is sometimes called a Batangas

Batangas

Batangas is a first class province of the Philippines located on the southwestern part of Luzon in the CALABARZON region. Its capital is Batangas City and it is bordered by the provinces of Cavite and Laguna to the north and Quezon to the east. Across the Verde Island Passages to the south is the...

 knife, after the Tagalog province of Batangas in the Philippines, where it is traditionally made. In the hands of a trained user, the knife blade can be brought to bear quickly using one hand.