What is "slipjoint"

A slipjoint knife is one of the most ubiquitous types of pocketknives.

A slipjoint knife consists of a handle with one or more folding blades. These blades are held in position by a strong "backspring" which biases them towards the open or closed position. Contrast this with the penny knife, which has no locking mechanism other than friction, or locking knives which mechanically lock the blade in position.