Existing or present on all sides.
Example:
The ambient lighting in the restaurant was low, but there was a bright candle at each table.
A scientist might measure how long it takes a heated substance to cool to the ambient temperature, the temperature of the surrounding air. Ambient light is the light that fills an area or surrounds something that is being viewed, like a television screen or a painting. A restaurant with low ambient light and candles at each table is probably trying for a romantic ambience, or atmosphere.