A bayonet is a knife

Knife

A knife is a cutting tool with an exposed cutting edge or blade, hand-held or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools...

, dagger

Dagger

A dagger is a fighting knife with a sharp point designed or capable of being used as a thrusting or stabbing weapon. The design dates to human prehistory, and daggers have been used throughout human experience to the modern day in close combat confrontations...

, sword

Sword

A sword is a bladed weapon used primarily for cutting or thrusting. The precise definition of the term varies with the historical epoch or the geographical region under consideration...

, or spike-shaped

Spike bayonet

A spike bayonet, also known as a pigsticker in informal contexts, is a blade attachment for a firearm taking the form of a pointed spike rather than a knife. Most early musket bayonets were of this type. From around 1800 knife- or sword style bayonets began to appear, which could also be wielded by...

 weapon

Weapon

A weapon, arm, or armament is a tool or instrument used with the aim of causing damage or harm to living beings or artificial structures or systems...

 designed to fit in, on, over or underneath the muzzle

Muzzle (firearm)

The muzzle of a firearm is the end of the barrel from which the projectile will exit.Precise machining of the muzzle is crucial to accuracy, because it is the last point of contact between the barrel and the projectile...

 of a rifle

Rifle

A rifle is a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves cut into the barrel walls. The raised areas of the rifling are called "lands," which make contact with the projectile , imparting spin around an axis corresponding to the...

, musket

Musket

A musket is a muzzle-loaded, smooth bore long gun, fired from the shoulder. Muskets were designed for use by infantry. A soldier armed with a musket had the designation musketman or musketeer....

 or similar weapon

Weapon

A weapon, arm, or armament is a tool or instrument used with the aim of causing damage or harm to living beings or artificial structures or systems...

, effectively turning the gun into a spear

Spear

A spear is a pole weapon consisting of a shaft, usually of wood, with a pointed head.The head may be simply the sharpened end of the shaft itself, as is the case with bamboo spears, or it may be made of a more durable material fastened to the shaft, such as flint, obsidian, iron, steel or...

. In this regard, it is an ancillary

Ancillary weapon

An ancillary weapon is a weapon used for secondary or auxiliary purpose, such as a bayonet. Such weapons are usually equipped with less firepower than the primary weapon. The usage of ancillary weapons most frequently occurs when the primary weapon becomes inoperable or ineffective due to shortage...

 close-quarter combat or last-resort weapon.

However, knife-shaped bayonets—when not fixed to a gun barrel—have long been utilized by soldiers in the field as general purpose cutting implements.
The origins of the bayonet are hazy, but appear to have begun in 17th century France.