Definition #1: Sound effects that sonically define the time and place of a location. (source: Larry Blake) also know as ambience and atmospheres.

Definition #2: Ambience helps establish the scene and works editorially to support the picture editing by, for example, staying constant across a picture cut to indicate to the audience that no change of space has occurred, but rather only a simple picture edit. Conversely, if ambience changes abruptly at a picture edit, an indication is made to the listener that the scene has also changed. (source: Tomlinson Holman of THX fame)

Definition #3:A continuous general sound that is indigenous to a given area where a film sequence is shot. Can be anything from traffic to surf or birds or crowd Walla. (source: M.P.S.E. - Motion Picture Sound Editors)