AirborneAirborne means carried by air, e.g. airborne particles, or carried in or on, and operating from aircraft, e.g. AWACS (Airborne Warning and Control System). An aircraft or helicopter is said to be airborne when it is in the air after taking off.
In particular, airborne (the airborne, airborne troops, etc.) are military units set up to be moved by aircraft and dropped into battle behind enemy lines. Thus they can be placed behind enemy lines, and have an ability to deploy almost anywhere with little warning. In addition the formations are limited only by the number and size of their aircraft, so given enough large aircraft a huge force can appear "out of nowhere" in minutes, an action referred to as vertical envelopment.
A complete airborne force consists of three types of units, paratroops, airlanding and airmobile. Paratroops are the original basis for the airborne force, but it was obvious from the start that a normal parachutist could carry enough equipment for very limited combat. To enhance their supplies the parachutist was sometimes followed into WW2 combat by the airlanding troops in gliders, and finally by the more conventional units in aircraft who fly into captured airfields, known as the airmobile.
The basic premise of the "airborne" is that they can arrive with such speed that a coherent defence cannot be mounted against them for some time. It is assumed that this tactical advantage cannot be sustained for very long, so effective airborne missions require the rapid advance of ground based troops in support.
The first reference to the use of what we would now call a paratroop force dates back to 1918 when in the midst of WWI Colonel Billy Mitchell suggested dropping elements of the US 1st Division behind German lines near Metz. It's somewhat unclear how this was to be achieved given the state of development of both the parachute and aircraft at the time.
After the war most of the progress in advanced military thinking took place in Italy and in addition to the jet engine, assault rifles, strategic bombing and a host of other "advances", they were also the first country to carry out a true paratroop drop in November 1927. Within a few years several battalions had been raised and were eventually formed into the two elite Folgore and Nembo divisions. Althogh these would go on to fight with distinction in WWII, they were never used in a parachute drop.
At about the same time the USSR was also experimenting with the idea, planning to eventually drop entire units complete with vehicles. To train enough experienced jumpers, parachute clubs were set up all over Russia with the aim of being able to transfer skilled members (or at least the men) into the armed forces if needed. The plan had progressed to the point that their large drops were demonstrated to foreign observers in 1936.
One of the observing parties, Germany, was particularly interested. In 1936 Major Immanns was ordered to set up a parachute school and was given a number of Junkers Ju 52 aircraft to train on. The military had already purchased large numbers of Junkers Ju 52 aircraft which were now modified (slightly) for use as paratroop transports in addition to their other duties.
Several groups within the German armed forces attempted to raise their own paratroop formations and there was some confusion all around. This changed when the Luftwaffe General Kurt Student was put in command of the effort, and the true power of the Fallschirmjager finally started to take form. Several operations were taken during the war (plus several others that were not carried out for a number of reasons) before Hitler forbade the airdrop after the very close battle on Crete. He felt that the main power of the paratroop was novelty, and now that the British had clearly figured out how to defend against them, there was no real point to using them any more.
Perhaps the most famous airborne operation of history is Operation Market Garden of September 1944, in which 35,000 troops were dropped 100 miles behind the German front lines in an attempt to capture a bridge over the Rhine at Arnhem. Three complete airborne divisions, the British 1st Airborne, and the US 82nd and 101st Airborne Divisions, were dropped at various points along Highway 69 in order to create a "carpet" over which the British XXX Corps could rapidly advance. German opposition was some three times that expected, including two under-strength but very experienced panzer divisions, and in the end the British 1st Airborne division was all but destroyed and the bridge remained in German hands.
In recent times the Mil 24 Hind, H-3 Sea King/Commando, H-46 Sea Knight, H-47 Chinook and H-53 Jolly Green Giant (or Sea Stallion) medium-lift helicopters have all been used to deploy soldiers or marines rapidly across battlefields (eg. in Afghanistan, the Falkland Islands/Malvinas, Iraq and Sierra Leone.)