What does the term “critical angle” mean, as applied to radio wave propagation?

When transmitting a radio signal skyward, the angle at which it leaves your antenna is called the takeoff angle. If this angle is too steep, the wave will pass straight through the ionosphere and into space instead of bending back toward the ground. The "critical angle" is the maximum or highest takeoff angle at which a radio wave of a given frequency will still be bent sufficiently by the ionosphere to return to Earth. Any angle higher than this critical threshold results in the signal escaping into space, while lower (shallower) angles will successfully reflect back, often traveling longer distances.