noun
1. : the least angle of incidence at which total reflection takes place when a ray of light or other electromagnetic radiation passes through one medium toward another that is less refracting
2. or critical angle of attack aeronautics : the angle of attack at which the flow about an airfoil changes abruptly with corresponding abrupt changes in the lift and drag, an airfoil possibly having two or more critical angles one of which usually corresponds to the angle of maximum lift
[s]critical.jpg[/s] [
critical angle: AOB, AO'B', AO''B'' angles of incidence; AOC refracted ray; AO''C'' totally reflected ray; AO'B' critical angle
]