A knife is a sharp-edged hand tool typically used for cutting. A minimal knife is a blade and a handle or some other method of gripping it. On Earth, knives have been used as tools and weapons since the Stone Age.
The first Earth knives were made of stone, chipped or ground to an edge, sometimes with a handle. Advances in technology led to blades made of bronze, iron, steel, and other exotic materials. Many cultures, such as the Klingons, had special knives for significant purposes such as eating, ceremonial practices, and assassinations.
In 2152, on a rescue mission to prevent Captain Jonathan Archer and Lieutenant Malcolm Reed from being hanged, Commander Charles Tucker III used a knife to cut the nooses from around their necks. (ENT: "The Communicator")
In 2154, Udar had a knife which had a serated blade. He was armed with the knife on planet Trialas IV when he was found there by an away team from Enterprise NX-01, led by Captain Archer. Udar charged Archer with the knife but Archer twisted Udar's wrist, causing him to drop the knife to the ground.
In a recreation room aboard the USS Enterprise in 2266, a knife was produced by a food synthesizer as part of a meal which Lieutenant junior grade Joe Tormolen obtained for himself. Tormolen, crazed by polywater intoxication, picked up the knife moments later, and then used it to threaten Hikaru Sulu and Kevin Riley. In an ensuing struggle, Tormolen eventually stabbed himself, accidentally, with the knife, an injury he subsequently died from. (TOS: "The Naked Time")
The Rakhari Croden told Constable Odo that he was shocked when the security officers one night came into his home and murdered his wives. He had no weapon except a short, slaughter knife that he used to cut the necks of fowl and birds he prepared for supper. It made the same sound when he cut the gullets of the security officers. (DS9: "Vortex")