Well... unless you're carrying a torch, lighter, flash or flashlight to light your way with, the light that's all around you right now is ambient light.

Usually used a term in photography to talk about the light that is available to the photographer before he/she adds additional suppliments.

Even more often referred to as a low level light that the photographer is forced (for whatever reason, from happenstance to choice) to shoot in without additional aid, as in:

It's a shame this picture didn't turn out, but I was working with ambient light..."

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Interchangeable with available light or existing light.