Army Cooperation Aviation
Army cooperation aviation and variations on the term, encompasses a variety of helicopters and (usually) light fixed-wing aircraft that are under the direct control of ground forces commanders, rather than an air force. The first such applications were with light aircraft used to observe the fall of artillery shells, to give a ground commander a literal "high-level view" of the battlefield, to carry small high-priority messages or supplies, etc. As helicopters became more common, due to their short range and use in ground forces tactics, they tended to be assigned to ground forces; long-range rescue and special operations helicopters might be under air forces or navies.
Small unmanned aerial vehicles (UAV) are complicating the situation, with some small, sensor-carrying UAVs being assigned to ground forces intelligence units rather than army aviation.
The term appears to have been used in the First World War.  While the fighters and bombers of the time were more glamorous, army cooperation aircraft carried out a wide range of missions, including Army cooperation aircraft performed a variety of diverse duties including photoreconnaissance, artillery spotting, and observation of enemy troop movements. Aircraft not having become enormously specialized, they also conducted ground strafing, and daylight tactical bombing.
Three different biplanes were the most common army cooperation aircraft of WWI: the British B.E.2c, the German Junkers J-I, and the British DeHavilland DH-4. Of the B.E.2c, it could only be said that the aircraft managed to be the worst in every category rated, but bureaucratic intertia kept it in service. The J-1, however, was ahead of its time in aerodynamic performance, but was difficult to produce.
Of these aircraft, only the DH-4, British designed and U.S. manufactured, remained in production after the war, and was in wide use into the next decade.
Interwar and Second World War
All the major powers developed army cooperation aircraft between the world wars, but their roles had to be reevaluated; they were not survivable if modern fighters shared the air.
After several unsuccessful attempts, the Westland Lysander was purpose-built for army cooperation; the problem was that army cooperation was ill-defined. Various observation and artillery fire direction and courier services seemed the basic requirement, along with short takeoff and landing. The aircraft was relatively large, as special message and supply delivery were seen as roles. 
For its original roles, the Lysander, as well as its German counterpart, the Henschel Hs 126, were deathtraps in the skies of Western Europe in 1940. Smaller, more agile fighters were given the reconnaissance role, but, later in the war, the Lysander found a niche. It could survive when the fighter threat was low, but it found a number of specialized activities not on the front line. While it could not land on water, it proved useful in search and rescue, being able to fly low and slow to spot downed crews, and large enough to drop life rafts. It carried out various support roles such as target towing, but was most important for special operations, carrying agents and supplies in and out of occupied Europe, flying at night. Its short-field handling was excellent in this role, and it was also used for special operations parachuting.
- Loftin, Laurence K., Jr., Chapter 2: Design Exploration, 1914-18; Army Cooperation Aircraft, Quest for Performance: The Evolution of Modern Aircraft, vol. I: THE AGE OF PROPELLERS
- Rickard, J (21 November 2007), "Westland Lysander", History Of War.org