angle of incidence

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The angle formed by a ray incident on a surface and a perpendicular to the surface at the point of incidence.

Wiktionary

1. n. 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.
2. n. 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.

GNU Webster's 1913

1. (Aëronautics) The angle between the chord of an aërocurve and the relative direction of the undisturbed air current.
2. n. the angle which a ray of light, or the line of incidence of a body, falling on any surface, makes with a perpendicular to that surface; also formerly, the complement of this angle.

WordNet 3.0

1. n. the angle that a line makes with a line perpendicular to the surface at the point of incidence