A utensil or a tool designed for cutting, consisting of a flat piece of hard material, usually steel or other metal (the blade), usually sharpened on one edge, attached to a handle. The blade may be pointed for piercing.
A weapon designed with the aforementioned specifications intended for slashing and/or stabbing and too short to be called a sword. A dagger.
verb
To cut with a knife.
To use a knife to injure or kill by stabbing, slashing, or otherwise using the sharp edge of the knife as a weapon.