Angle of flash coverage
From The Glossary of Digital Photography
As the name implies, a number that expresses the width of the light generated by a flash. This number is often described as a focal length, as with a lens. This similarity makes it easy to see if the angle of flash coverage of a flash is large enough to cover the view of a particular lens. As long as the angle of flash coverage is larger than the focal length of the lens being used, there will be a minimal amount of light fall off on the edges of the image. Some flashes have a variable angle of flash coverage that uses a lens to widen or narrow the angle. Other flashes have a flip-down lens that converts the flash to a wider angle for use with wide-angle lenses.
