angle of incidence

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

• noun The angle formed by a ray or wave incident on a surface and a line perpendicular to the surface at the point of incidence.

from the GNU version of the Collaborative International Dictionary of English.

• (Aëronautics) The angle between the chord of an aërocurve and the relative direction of the undisturbed air current.

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

• noun optics, physics The angle that a straight line, ray of light, etc., meeting a surface, makes with a normal to the surface at the point of meeting.
• noun aeronautics The angle, usually fixed, between the chord line of a wing or horizontal stabilizer and the axis of the fuselage, measured at the root. In UK called the angle of attack.

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

• noun the angle that a line makes with a line perpendicular to the surface at the point of incidence