453
The angle that a ray of sunlight makes with a line perpendicular to the surface of the solar panel. When the solar panel surface directly faces the sun the solar angle of incidence is zero. If the solar panel surface is parallel to the sun the angle of incidence is 90°.
The angle of incidence with vary throughout the day. On fixed mounted solar panels the angle of incidence could go from 90° at sunrise to zero to 90° again at sunset. Tracking mounts keep the angle of incidence as close to zero as possible but at the extra expensive of the tracking mechanisms.