Digital SLR Camera Basics
A duplicate image of the sun, car headlights, or other bright light source at a different position in the frame from the original light source. Ghosting is distinct from flare, in which light from a bright light source is scattered or diffused over an area of the frame without creating a duplicate image.
A numeric expression of flash power. The higher the guide number, the greater the power of the flash and the greater the distance at which the flash can provide sufficient illumination for optimal exposure.