The Paris Gun was the name given to a type of German long-range siege gun, several of which were used to bombard Paris during World War I. They were in service from March to August 1918. When the guns were first employed, Parisians believed they had been bombed by a high-altitude Zeppelin, as the sound of neither an airplane nor a gun could be heard. They were the largest pieces of artillery used during the war by barrel length, and qualify under the (later) formal definition of large-calibre artillery. Also called the "Kaiser Wilhelm Geschütz", they were often confused with Big Bertha, the German howitzer used against Belgian forts in the Battle of Liège in 1914; indeed, the French called them by this name as well. They were also confused with the smaller "Langer Max" cannon, from which they were derived. Although the famous Krupp-family artillery makers produced all these guns, the resemblance ended there.

The formal definition of large-calibre artillery used by the United Nations Register of Conventional Arms (UNROCA) is "guns, howitzers, artillery pieces, combining the characteristics of a gun, howitzer, mortar, or rocket, capable of engaging surface targets by delivering primarily indirect fire, with a calibre of 76.2 mm (3.00 in) and above". This definition, shared by the Arms Trade Treaty and the Treaty on Conventional Armed Forces in Europe, is updated from an earlier definition in United Nations General Assembly Resolution 46/36L, which set a threshold of 100 mm (3.9 in). Several grammatical changes were made to that latter in 1992 and the threshold was lowered in 2003 to yield the current definition, as endorsed by UN General Assembly Resolution 58/54.