A baton (also truncheon, nightstick, billy club, billystick, cosh, lathi, or simply stick) is a roughly cylindrical club made of wood, rubber, plastic, or metal. It is carried as a compliance tool and defensive weapon by law-enforcement officers, correctional staff, security guards and military personnel. The name baton comes from the French bâton (stick), derived from Old French Baston, from Latin bastum.

As a weapon a baton may be used defensively (to block) or offensively (to strike, jab, or bludgeon), and it can aid in the application of armlocks. The usual striking or bludgeoning action is not produced by a simple and direct hit, as with an ordinary blunt object, but rather by bringing the arm down sharply while allowing the truncheon to pivot nearly freely forward and downward, so moving its tip much faster than its handle. Batons are also used for non-weapon purposes such as breaking windows to free individuals trapped in a vehicle, or turning out a suspect's pockets during a search (as a precaution against sharp objects).

Some people other than law enforcement officers use batons as weapons because of their simple construction and easy concealment. The use or carrying of batons or improvised clubs by people other than law enforcement officers is restricted by law in many countries.

In the Victorian era, police in London carried truncheons about one foot long called billy clubs. According to the Online Etymology Dictionary, this name was first recorded in 1848 as slang for a burglars' crowbar. The meaning "policeman's club" is first recorded 1856. The truncheon acted as the policeman's 'Warrant Card' as the Royal Crest attached to it indicated the policeman's authority. This was always removed when the equipment left official service (often with the person who used it).

The Victorian original has since developed into the multitude of varieties available today. The typical truncheon is a straight stick made from wood or a synthetic material, approximately 32 mm (1+1⁄4 in) in diameter and 460–910 mm (18–36 in) long, with a fluted handle to aid in gripping. Truncheons are often ornamented with their organizations' coats of arms. Longer truncheons are called "riot batons" because of their use in riot control. Truncheons may have developed as a marriage between the club or military mace and the staff of office/sceptre.

Straight batons of rubber have a softer impact. Some of the kinetic energy bends and compresses the rubber and bounces off when the object is struck. Rubber batons are not very effective when used on the subject's arms or legs, and can still cause injury if the head is struck. That is why most police departments have stopped issuing them. ^{[citation needed]} The Russian police standard-issue baton is rubber, except in places such as Siberia, where it can be cold enough that the rubber may become brittle and break if struck. The traffic baton is red to make it more visible as a signaling aid in directing traffic. In Russia, traffic batons are striped in black and white for the same reason, and in Sweden they are white. Until the mid-1990s, British police officers carried traditional wooden truncheons of a sort that had changed little from Victorian times. Since the late 1990s, the collapsible baton is issued except for public order duties, where a fixed, acrylic baton is used. Side-handled batons were issued for a while, but fell out of favour.

The New York City Police Department used to use two kinds of batons depending on the time. The one for daytime was called a day-stick and was 280 mm (11 in) in length. Another baton, that was used at night, was 660 mm (26 in) long and called a night-stick, which is the origin of the word nightstick. The night-stick was longer so it could provide extra protection which was thought to be necessary at night.

In modern police training, the primary targets are large nerve clusters, such as the common peroneal nerve in the mid-thigh and large, easily targetable muscle groups, such as the quadriceps and biceps. The baton is swung in fast, "snapping" strikes to these areas, sometimes only making contact with the tip. Taken together, these are intended to impair the subject's ability to continue advancing (by striking the leg) or attack (by striking the arm) by causing transitory neurapraxia (temporary muscle pain, spasm and paralysis due to nerve injury). Modern systems strictly prohibit hitting the skull, sternum, spine, or groin unless such an attack is conducted in defense of life, with many jurisdictions considering this deadly force.