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..."
Interchangeable with
available light or
existing light.