A knife (ナイフ) is a type of blade weapon. It is generally small, although its most distinguishing characteristic is that it has one sharp edge.
Launch uses knifes when cooking, even chasing Krillin with a butcher's knife on Training Island after she turned into her evil form when he asked a brewsky for Master Roshi. Launch then sneezes again and, in doing so, she throws the knife in the air and it lands right in front of a frightened Krillin. Bulma and Oolong also uses knifes when cooking, and many of the other Dragon Ball Gang member use them when eating.
General Blue uses a small knife to threaten Arale Norimaki when holding her hostage and demand that Goku surrender during their encounter at Penguin Village. Arale's creator Senbei Norimaki attempts to warn Blue that using a knife to threaten Arale is a very bad idea, which Arale soon confirms by licking the blade.
After his conversion into a Cyborg, Mercenary Tao has his left arm conceal a blade, which he uses to slice Tien Shinhan in the chest in their match in the 23rd World Martial Arts Tournament, such an action leading to his being disqualified from the tournament due to weapons usage being banned in the tournament.
In Dragon Ball Z: Broly - The Legendary Super Saiyan, during Paragus's flashback to how King Vegeta nearly had both himself and his son Broly executed, it was shown that the latter was stabbed with a dagger by a Saiyan guard, which nearly killed him.