critical angle

Engineering, Natural sciences, Optics, Physical sciences, Physics The angle of incidence beyond which light traveling through a medium experiences total internal reflection upon encountering a boundary with a less refractive medium.

Aeronautics, Aerospace, Aviation, Business, Engineering, Natural sciences, Physical sciences The angle of attack beyond which an airfoil stalls, experiencing a decrease in lift and increase in drag due to separation of airflow from the upper surface of the airfoil.

Architecture The angle beyond which a ramp or a set of stairs is considered uncomfortable or unsafe.