critical angle

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the smallest angle of incidence for which light is totally reflected