Generally thought of as the character or quality of some environment. In audio we specifically mean the acoustical (sonic) characteristics of a space, including everything from the size of the space to what type of sounds are a normal part of it. For example: a big auditorium may, as part of its sonic character, have a large HVAC (Heating, Ventilation, and Air Conditioning) system that runs, which provides a constant background noise level. Any noise existing or injected into a space will of course be acted upon by the space based on things like size, shape, and the various surfaces that reflect sound. Humans have a great ability to discern a lot about an environment from these aural cues. As an experiment put up a microphone in some different rooms of your house or apartment and make recordings. Listen to them later at an exaggerated volume so you can really hear the ambient noise level as it is being acted upon by the room acoustics. You will most likely be able to identify each room by its sound.