Ricochet firing is the firing of artillery

Artillery

Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...

 at a low angle

Angle

In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...

 and non-maximal power so as to cause the shells

Shell (projectile)

A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage sometimes includes large solid projectiles properly termed shot . Solid shot may contain a pyrotechnic compound if a tracer or spotting charge is used...

 to skip across the ground.

In traditional artillery tactics, either a cannon

Cannon

A cannon is any piece of artillery that uses gunpowder or other usually explosive-based propellents to launch a projectile. Cannon vary in caliber, range, mobility, rate of fire, angle of fire, and firepower; different forms of cannon combine and balance these attributes in varying degrees,...

, a howitzer

Howitzer

A howitzer is a type of artillery piece characterized by a relatively short barrel and the use of comparatively small propellant charges to propel projectiles at relatively high trajectories, with a steep angle of descent...

 or occasionally a mortar

Mortar (weapon)

A mortar is an indirect fire weapon that fires explosive projectiles known as bombs at low velocities, short ranges, and high-arcing ballistic trajectories. It is typically muzzle-loading and has a barrel length less than 15 times its caliber....

 would fire a shot that would just clear the outer parapet

Parapet

A parapet is a wall-like barrier at the edge of a roof, terrace, balcony or other structure. Where extending above a roof, it may simply be the portion of an exterior wall that continues above the line of the roof surface, or may be a continuation of a vertical feature beneath the roof such as a...

 of a fortification, and then would bounce or ricochet inside the fortification's bounds. One of the primary purposes of Ricochet Fire was to dismount artillery and guns positioned on an enemy's fortifications. Additionally, Ricochet fire could also be used to cause chaos behind an enemy army's fortifications, as the ricocheting cannonball would devastate logistical structures not fortified to withstand cannon fire. The first European use of Ricochet Fire has been accredited to the Marquis de Vauban during the siege of Namur

Namur (city)

Namur is a city and municipality in Wallonia, in southern Belgium. It is both the capital of the province of Namur and of Wallonia....

, although it is noted that Turkish artillery divisions were using ricochet fire as early as 1608.

Sources;

http://civilwarfortifications.com/dictionary/xgf-006.html