Definitions for "Existing light"
see available light
Available light, includes all natural lighting from moonlight to sunshine; and for photographic purposes, existing light is also the light that is already on the scene. Therefore it takes in: room lamps, fluorescent lamps, neon signs, candles, daylight through windows, and artificially illuminated night scenes.
The use of the existing ambient lighting in a location without augmenting or replacing it with artificial lighting be that flash or tungsten or HMI.