What is "butterfly knife"

n. a folding pocket knife with two handles counter-rotating around the tang such that, when closed, the blade is concealed within grooves in the handles.

A butterfly knife, also known as a fan knife and in the Philippines as the balisong, is a folding pocket knife. Its distinction is two handles counter-rotating around the tang such that, when closed, the blade is concealed within grooves in the handles. It is sometimes called a Batangas knife, after the Tagalog province of Batangas in the Philippines, where it is traditionally made.

The balisong was commonly used by Filipino people, especially those in the Tagalog region, as a self-defense and pocket utility knife. A common stereotype is that a Batangueño carries one everywhere he or she goes. Hollow-ground balisongs were also used as straight razors before conventional razors were available in the Philippines. In the hands of a trained user, the knife blade can be brought to bear quickly using one hand. Manipulations, called "flipping" or "fanning", are performed for art or amusement. Blunt versions of these knives, called "trainers", are for sale to practice tricks without the risk of injury.

The knife is now illegal or restricted in many countries, often under the same laws and for the same reasons that switchblades are restricted, and in their country of origin they are no longer as common in urban areas as they were.