Ambient light
A constant base illumination added to every surface to fake the indirect light a scene would otherwise lack.
The simplest term in the lighting equation: a fixed colour added everywhere regardless of angle or distance, so shadowed surfaces don't go pitch black. It is a crude approximation of global indirect light — one number instead of real bounces — and looks flat for exactly that reason.
Early real-time engines had nothing else; a scene got an ambient level and made do. The flat constant was later replaced by direction-dependent variants — ambient cubes, spherical harmonics, light probes — and eventually by real global illumination and ambient occlusion, which darkens where surfaces meet. Plain constant ambient still survives as a cheap fallback.