an aerodynamic condition in which the smooth flow of air has broken away from the upper surface of an airfoil, and the flow is turbulent, decreasing the amount of lift produced

Aircraft technical dictionary, Jeppessen, 1997

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Note added at 30 mins (2007-08-30 12:11:32 GMT)
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http://en.wikipedia.org/wiki/Stall_(flight)#Stalling_an_aero...
Stalling an aeroplane

An aeroplane can be made to stall in any pitch attitude or bank angle or at any airspeed but is commonly practiced by reducing the speed to the unaccelerated stall speed, at a safe altitude. Unaccelerated (1g) stall speed varies on different aeroplanes and is represented by colour codes on the air speed indicator. As the plane flies at this speed the angle of attack must be increased to prevent any loss of altitude or gain in airspeed (which corresponds to the stall angle described above). The pilot will notice the flight controls have become less responsive and may also notice some buffeting, an aerodynamic vibration caused by the airflow starting to detach from the wing surface.

In most light aircraft, as the stall is reached the aircraft will start to descend (because the wing is no longer producing enough lift to support the aeroplane's weight) and the nose will pitch down. Recovery from this stalled state usually involves the pilot decreasing the angle of attack and increasing the air speed, until smooth air flow over the wing is resumed. Normal flight can be resumed once recovery from the stall is complete. The manoeuvre is normally quite safe and if correctly handled leads to only a small loss in altitude. It is taught and practised in order to help pilots recognize, avoid, and recover from stalling the aeroplane.

The most common stall-spin scenarios occur on takeoff (departure stall) and during landing (base to final turn) because of insufficient airspeed during these manoeuvres. Stalls also occur during a go-around manoeuvre if the pilot does not properly respond to the out-of-trim situation resulting from the transition from low power setting to high power setting at low speed. Stall speed is increased when the upper wing surfaces are contaminated with ice or frost creating a rougher surface.

A special form of asymmetric stall in which the aircraft also rotates about its yaw axis is called a spin. A spin will occur if an aircraft is stalled and there is an asymmetric yawing moment applied to it. This yawing moment can be aerodynamic (sideslip angle, rudder, adverse yaw from the ailerons), thrust related (p-factor, one engine inoperative on a multi-engine non-centreline thrust aircraft), or from any number of possible sources of yaw.

Since most aircraft have an engine, some confusion exists between an aerodynamic versus engine stall. Many people seem to believe that an aircraft will drop out of the sky as soon as the engine stops in flight. In reality, the pilot can simply lower its nose to generate enough airspeed to maintain lift over the wings and so prevent a stall. The aircraft will then descend at a steady airspeed. The pilot then has time to find a suitable landing area or to restart the engine.

Put differently, all powered aircraft (even the biggest ones) become gliders when they lose all thrust. There have been cases of airliners running out of fuel at high altitude that landed successfully at airports a hundred kilometres away. However the distance which an aircraft can glide is directly related to the airspeed, but most of all the density altitude which the aircraft is at. The Gimli Glider is a celebrated example.

Stalls can occur at higher speeds if the wings already have a high angle of attack. Attempting to increase the angle of attack at 1g by moving the control column back simply causes the aircraft to rise. However the aircraft may experience higher g, for example when it is pulling out of a dive. In this case, the wings will already be generating more lift to provide the necessary upwards acceleration and so there will be higher angle of attack. Increasing the g still further, by pulling back on the control column, can cause the stalling angle to be exceeded even at a high speed. High speed stalls produce the same buffeting characteristics as 1g stalls and can also initiate a spin if there is also any yawing.

[edit] Symptoms of an approaching stall

One symptom of an approaching stall is slow and sloppy controls. As the speed of the aeroplane decreases approaching the stall, there is less air moving over the wing and therefore less will be deflected by the control surfaces (ailerons, elevator and rudder ) at this slower speed. Some buffeting may also be felt from the turbulent flow above the wings as the stall is reached. However during a turn this buffeting will not be felt and immediate action must be taken to recover from the stall. The stall warning will sound, if fitted, in most aircraft 5 to 10 knots above the stall speed.
Stalling characteristics

Different aircraft types have different stalling characteristics. A benign stall is one where the nose drops gently and the wings remain level throughout. Slightly more demanding is a stall where one wing stalls slightly before the other, causing that wing to drop sharply, with the possibility of entering a spin. A dangerous stall is one where the nose rises, pushing the wing deeper into the stalled state and potentially leading to an unrecoverable deep stall. This can occur in some T-tailed aircraft where the turbulent airflow from the stalled wing can blanket the control surfaces at the tail.