Similar to a room's acoustic although generally the term only refers to small spaces.
Ambience is the acoustic sound generated by a room. With the judicious use of microphone techniques, the ambience of a room in which an instrument is being recorded can be picked up and added to the direct sound of the instrument. You can alter the ratio of direct sound and abience by changing the distance of the microphone to the source, and also with the polar patern of the microphone. Adding Ambience gives a greater sense of space within the recorded sound and can lead to a very natural-sounding stereo image.
Sound Engineers use the word Ambience to refer to Reverbs with either a short time, or one made predominantly of early reflections and little or no tail. Short Reverbs are useful for adding a sence of space and depth without the rhythmic smearing caused by longer reverbs.