Definitions for "Guide number"
Number used to indicate the light output of a flash unit, affected by the type of reflector used
A number that relates the output of electronic flash when used with a particular speed film. The higher the guide number, the more the light output. Guide numbers, or GN serve as a way to calculate aperture when shooting flash in manual exposure mode. Dividing distance into guide number gives the aperture: For example, a flash with a guide number of 56 (with ISO 100 film) would give a correct exposure at 10 feet with an aperture of f/5.6. With the state of today's automatic exposure flashes, guide numbers today are mostly useful for comparing the relative power of one flash to another.
A numerical valuation for the power of a flash. Used to determine the aperture that will produce correct flash exposure with a given ISO and distance from flash to subject. Guide Number = f/stop x Distance.