A field gun is an artillery piece. Originally the term referred to smaller guns that could accompany a field army on the march and when in combat could be moved about the battlefield in response to changing circumstances (field artillery), as to opposed guns installed in a fort (garrison artillery/coastal artillery), or to siege cannon or mortars which were too large to be moved quickly, and would be used only in a prolonged siege. Perhaps the most famous use of the field gun in terms of advanced tactics was Napoleon's use of very large wheels on the guns that allowed them to be moved quickly even during a battle. By moving the guns from point to point during the battle, enemy formations could be broken up to be handled by the infantry wherever they were massing, dramatically increasing the overall effectiveness of the infantry.
Contributions by GraemeLeggett, Sgm2000, and 220.127.116.11.