Cutoff Angle (Lighting) : With regard to an outdoor lighting luminaire, the angle between the vertical axis and the first line of sight at which the bare light source is not visible
Young & Company Electric Power Products and Related Engineering Services, Glossary of Electrical Terms, http://www.youngco.com/young2.asp?ID=4&Type=3, CA., USA 2001. (Electr Sfty - Amer) {514}