critical angle

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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. n. The angle of attack of an airfoil at which airflow abruptly changes, causing changes in the lift and drag of an aircraft.

Wiktionary

1. n. physics The smallest angle of incidence for which light is totally reflected from a boundary between one medium and a less refractive one
2. n. aerodynamics The smallest angle of attack of an airfoil at which lift suddenly decreases and drag suddenly increases
3. n. architecture The smallest angle of a ramp or stairs considered uncomfortable or unsafe

GNU Webster's 1913

1. adj. (Optics) that angle of incidence of a luminous ray at which it is wholly reflected, and no portion of it transmitted. The sine of this angle is the reciprocal of the refractive index of the medium.

WordNet 3.0

1. n. the smallest angle of incidence for which light is totally reflected