Angle of incidence
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An angle of incidence is the angle between a beam incident on a surface and the normal (line perpendicular to the surface at the point of incidence). The beam can be formed by any wave: optical, acoustical, microwave, X-ray etc. In Fig.1 the red line representing a ray makes an angle θ with the normal (dotted line).
Another common usage is in aviation, where it refers to the angle between the wing's chord (aircraft) and the longitudinal axis of an aircraft (a fixed value). Fig.2 shows a side view of part of an aeroplane. The wing (dotted blue line) makes an angle a with the fuselage (solid blue line). The wings are typically mounted at a small positive angle of incidence, to allow the fuselage to be "flat" to the airflow in normal cruising flight. Angles of incidence of about 6° are common on most general aviation designs. Another term for angle of incidence in this context is rigging angle. It should not be confused with the angle of attack, which is the angle the wing chord presents to the airflow in flight. Note that some ambiguity in this terminology exists, as some engineering texts that focus solely on the study of airfoils and their medium may use either term when referring to angle of attack.