critical angle

1. The smallest angle of incidence at which a light ray passing from one medium to another less refractive medium can be totally reflected from the boundary between the two.
2. The angle of attack of an airfoil at which airflow abruptly changes, causing changes in the lift and drag of an aircraft.

In an optical fiber, the maximum angle from the axis at which light can remain confined within the core for "total internal reflection." In contrast to the "acceptance angle," which is measured outside the fiber, the critical angle is a measurement within the fiber. See numerical aperture.

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An angle of pitch of stairs or a ramp which is considered uncomfortable and unsafe if exceeded; this angle is 50° for stairs and 20° for ramps.

The noun has one meaning:

Meaning #1: the smallest angle of incidence for which light is totally reflected