A cannon (plural either cannons or cannon) is a large-caliber gun classified as a type of artillery, which usually launches a projectile using explosive chemical propellant. Gunpowder ("black powder") was the primary propellant before the invention of smokeless powder during the late 19th century. Cannons vary in gauge, effective range, mobility, rate of fire, angle of fire and firepower; different forms of cannon combine and balance these attributes in varying degrees, depending on their intended use on the battlefield. A cannon is a type of heavy artillery weapon. The word cannon is derived from several languages, in which the original definition can usually be translated as tube, cane, or reed.

The earliest known depiction of cannons may have appeared in Song dynasty China as early as the 12th century; however, solid archaeological and documentary evidence of cannons do not appear until the 13th century. In 1288, Yuan dynasty troops are recorded to have used hand cannons in combat, and the earliest extant cannon bearing a date of production comes from the same period. By the end of the 14th century, cannons were widespread throughout Eurasia.

Cannons were used primarily as anti-infantry weapons until around 1374, when large cannons were recorded to have breached walls for the first time in Europe. Cannons featured prominently as siege weapons. In 1464 a 16,000 kg (35,000 lb) cannon known as the Great Turkish Bombard was created in the Ottoman Empire. Cannons as field artillery became more important after 1453 when cannons broke down the walls of the Roman Empire's capital, with the introduction of limber, which greatly improved cannon maneuverability and mobility. European cannons reached their longer, lighter, more accurate, and more efficient "classic form" around 1480. This classic European cannon design stayed relatively consistent in form with minor changes until the 1750s.

In the modern era, the term cannon has fallen into decline, replaced by guns or artillery, if not a more specific term such as howitzer or mortar, except for high-caliber automatic weapons firing bigger rounds than machine guns, called autocannons.

The word cannon is derived from the Old Italian word cannone, meaning "large tube", which came from the Latin canna, in turn originating from the Greek κάννα (kanna), "reed", and then generalised to mean any hollow tube-like object. The word has been used to refer to a gun since 1326 in Italy and 1418 in England. The plural forms cannons and cannon are both correct.

The cannon may have appeared as early as the 12th century in China, and was probably a parallel development or evolution of the fire-lance, a short ranged anti-personnel weapon combining a gunpowder-filled tube and a polearm. Projectiles such as iron scraps or porcelain shards, mixed together with the gunpowder ("co-viative"), were placed in fire lance barrels at some point, and eventually, the paper and bamboo materials of fire lance barrels were replaced by metal.

The earliest known depiction of a cannon is a sculpture from the Dazu Rock Carvings in Sichuan dated to 1128, however, the earliest archaeological samples and textual accounts do not appear until the 13th century. The primary extant specimens of cannon from the 13th century are the Wuwei Bronze Cannon dated to 1227, the Heilongjiang hand cannon dated to 1288, and the Xanadu Gun dated to 1298. However, only the Xanadu gun contains an inscription bearing a date of production, so it is considered the earliest confirmed extant cannon. The Xanadu Gun is 34.7 cm (13.7 in) in length and weighs 6.2 kg (14 lb). The other cannons are dated using contextual evidence. The Heilongjiang hand cannon is also often considered by some to be the oldest firearm since it was unearthed near the area where the History of Yuan reports a battle took place involving hand cannons. According to the History of Yuan, in 1288, a Jurchen commander by the name of Li Ting led troops armed with hand cannons into battle against the rebel prince Nayan.

Chen Bingying argues there were no guns before 1259, while Dang Shoushan believes the Wuwei gun and other Western Xia era samples point to the appearance of guns by 1220, and Stephen Haw goes even further by stating that guns were developed as early as 1200. Sinologist Joseph Needham and renaissance siege expert Thomas Arnold provide a more conservative estimate of around 1280 for the appearance of the "true" cannon. Whether or not any of these are correct, it seems likely that the gun was born sometime during the 13th century.