A type of folding knife, usually larger than ordinary locking-blade knives but rarely more than a few inches long, constructed of two pivoting, hollow handles attached to a solid blade. When not in use, the handles fold back over the blade, which is stored within the handle cavities; when in use, handles latch together opposite the blade. Like switchblades, butterfly knives can be used for any task performed by an ordinary locking-blade knife but are better known for their intimidation value.

It is possible to open a butterfly knife one-handed with practice, but it's not nearly as simple as opening a locking-blade knife with a thumb stud. It is done by forcefully flipping the blade and one handle around the axis defined by the pivot joining the blade and the other handle; done properly, it looks quite badass.

There are a few problems with the design; for example, the pivots often loosen, causing the blade to rattle or become slightly loose. In poorly made examples, this can cause the blade to contact other components of the knife, scratching or chipping it. Butterfly knives are also a bit more dangerous than average to the user, due to the opening method; it's fairly easy to lose control of the knife while opening it, with unpredictable results.

Butterfly knives are illegal in the U.S. states of Massachusetts and California, as well as in the UK and Australia. Pylon informs me that they have also been banned in New York; DWarrior tells of an similar law in New Jersey.

See also balisong.

In addition to the states of Massachusetts and California, the butterfly knife was almost illegal in the state of Oregon, which forbids weapons which can be opened with the force of inertia or gravity alone. However, in a 1980's court case, it was successfully argued that movement of the thumb required to complete the opening manuever constitutes an element of skill, and moves the balisong out of the "Gravity Weapons" category.

Nonetheless, the butterfly knife has a bad reputation as a result of its being illegal in two states, and is commonly (mistakenly) believed to be illegal in all the states of the union.