Ambient Light traditionally refers to any form of light that the photographer did not add to their image, and can include both “natural” ambient sunlight, or “artificial” ambient light such as a nearby street light, lamp, or similar.
Ambient light means the light that is already present in a scene, before any additional lighting is added. It usually refers to natural light, either outdoors or coming through windows etc. It can also mean artificial lights such as normal room lights.