http://w3n-a.blogspot.com/2008/12/stalls.html

A stall occurs when the smooth airflow over the airplane's wing is
disrupted, and the lift degenerates rapidly. This is caused when the
wing exceeds its critical angle of attack. This can occur at any
airspeed, in any attitude, with any power setting.

The practice of stall recovery and the development of awareness of
stalls are of primary importance in pilot training. The objectives in
performing intentional stalls are to familiarize the pilot with the
conditions that produce stalls, to assist in recognizing an
approaching stall, and to develop the habit of taking prompt
preventive or corrective action.

Intentional stalls should be performed at an altitude that will
provide adequate height above the ground for recovery and return to
normal level flight. Though it depends on the degree to which a stall
has progressed, most stalls require some loss of altitude during
recovery. The longer it takes to recognize the approaching stall, the
more complete the stall is likely to become, and the greater the loss
of altitude to be expected.

http://w3n-a.blogspot.com/2008/12/stalls.html