Angle of incidence

angle that a line makes with a perpendicular to that surface at the point of meeting

Angle of incidence is a measure of deviation of something from "straight on", for example in the approach of a ray to a surface, or the direction of an airfoil with respect to the direction of an airplane.
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Noun
1. the angle that a line makes with a line perpendicular to the surface at the point of incidence
(synonym) incidence angle
(hypernym) angle
(hyponym) angle of attack

The angle between an incident ray and the normal to a reflecting or refracting surface. (188 )

the angle between a ray of light incident on a surface and a line drawn normal to the surface.