Wikipedia
Gun-howitzer (also referred to as gun howitzer) is a type of artillery weapon that is intended to fulfill both the role of ordinary cannon or field gun, and that of a howitzer. It is thus able to convey both direct and indirect fire.
To be able to serve as a howitzer, gun-howitzers are typically built to achieve up to 60—70° of elevation. For effective direct fire, the gun-howitzers typically employ a fairly long barrel, usually not shorter than 30 calibres. Its ammunition also has a high muzzle velocity and often large calibre (often exceeding 120 mm).
Historically the first gun-howitzer was the French canon obusier of 19th century. The smooth-bore Canon obusier de 12 was a versatile weapon that quickly replaced both ordinary cannons and howitzers in French service, and became one of the basic types of artillery used by both sides of the American Civil War. Owing to their versality, gun-howitzers gained prominence in the period leading to World War II as a more flexible weapon than ordinary howitzers and were adopted by armies of both the Allies (for instance the Soviet ML-20 152 mm M1937 and British (88 mm) Ordnance QF 25-pounder) and the Axis (German 10.5 cm leFH 18).