A Civil War British Bayonet as shown here

A bayonet (from French baïonnette) is a knife, dagger, sword, or spike-shaped weapon designed to fit in, on, over or underneath the muzzle of a rifle, musket or similar weapon, effectively turning the gun into a spear. In this regard, it is an ancillary 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. These Bayonets were mostly seen during the events of World War I and II and were mostly seen by the United States, Soviet, China, United Kingdom and Germany.

Throught the years of fighting, many Bayonet varriants released out for certain rifles such as bolt-action rifles, AK-47s and M16s and mostly can be rare to find any older Bayonet variant these days. 

For Use in Infectionous Apocalypse

Bayonets are both recommended and not recommended for use in killing zombies. It is recommended for only zombie assassinations or backup/last stand. It is not recommended because it is impossible to kill a infected zombie with a bayonet (depends on what bayonet variant it is). The eye socket and the soft pallete (through the jawbone and upwards) are the only entry points that can damage the brain, and they require expert precision to strike under duress. Despite its weakness it is worth having, for they can eliminate close-range zombies without wasting ammunition. With a long bayonet, one can stab not all the way through the zombie in a vital area such as the head on neck, and then fire a small number of shots, which is good for picking off small groups or bandits.

One can be improvised by taping/glueing any type of knife (Kitchen, Combat, Hunting, ETC) to the bottom of a gun for their own version of a Bayonet-styled weapon of choice.