http://w3n-a.blogspot.com/2008/11/introduction.html

The maintenance of lift and control of an airplane in flight requires
a certain minimum airspeed. This critical airspeed depends on certain
factors, such as gross weight, load factors, and existing density
altitude. The minimum speed below which further controlled flight is
impossible is called the stalling speed. An important feature of pilot
training is the development of the ability to estimate the margin of
safety above the stalling speed. Also, the ability to determine the
characteristic responses of any airplane at different airspeeds is of
great importance to the pilot. The student pilot, therefore, must
develop this awareness in order to safely avoid stalls and to operate
an airplane correctly and safely at slow airspeeds.

http://w3n-a.blogspot.com/2008/11/introduction.html