Basic Information
Not to be confused with a canon which is a kind of travelling padre.
An ordnance class gun of calibre greater than or equal to 20mm from the most primitive muzzle-loading black powder weapons to cutting edge technology. This was not always so straight forward.
In the 16th -17th century ordnance (all of it then muzzle loading and black powder, earlier experiments in breech loading being more or less extinct) consisted of a bewildering array of sakers, minions, falconets, culverins, demi- regular and great cannons and many other specific terms for weapons of a specific shot mass, calibre, materials of construction and relative barrel length, some of which meant different things in different nations. To most people of this period a cannon was a moderately long-form iron gun with a shot mass of ~42lb/19kg. This sort of diversity lead to obvious logistic challenges, as well as the risks inherent in having radically different powder charges to hand for different guns of the same calibre … eventually there was a common technical convergence onto a few designs of gun that were then built in different calibres to deliver different masses of shot and the term "cannon" was applied to all of them (and so, from the 18th century onwards, a six pounder cannon was more or less a scaled down version of a 32 pounder cannon. The original 42 pound cannon, ironically, became marginal as it was usually too heavy to be effectively moved and handled with the technology of the time). Divergence re-started again almost as soon as a consensus appeared, but from that point on the name, at least, appears to have stuck.
In strictest terms a cannon should also only fire at elevations between zero and forty-five degrees from the horizontal - becoming a howitzer if it fires between zero and ninety and a mortar if it is limited to elevations of between forty five and ninety. This definition also went by the board rather with the invention of anti-aircraft mounts and is not much observed in the modern era - although the term howitzer is still used for cannon capable of high-angle fire. Speaking of anti-aircraft weapons some sources suggest that a cannon should not be capable of fully automatic fire, but given the large number of automatic cannon that are, or have been, in service over the years this distinction seems to have gone by the board.
Can overlap with heavy weapons in some cases - such as the French M1916 37mm Infantry Gun (derivatives of which were issued by the Japanese and US Forces) or some of the heavy anti-tank rifles common in WW2 such as the Japanese Type 97 or the Solothurn S-18/1000 but should never be applied - except euphemistically - to small arms. If in doubt, these cases are referred back to the idea that a cannon is ordnance and as such a man-portable weapon is not a cannon (even if called a "hand-cannon").
Can also apply to directed energy weapons in a sci-fi setting.
See also the hand cannon and the concept of the cannon lock.