Definition #1: Any sound or noise important to the action or background of a scene. They come from four sources: (1) production (those recorded during filming); (2) sync effects (Foley); (3) wild tracks; and (4) a library of sound effects saved over a period of years and kept in stock for reuse.(source: M.P.S.E. - Motion Picture Sound Editors)

*also known as FX.

Definition #2: Sound Effects Editing: … the process of choosing sounds, then cutting the elements or units into synchronisation with the picture, and delivering them with the cue sheet to the dubbing stage for premixing. (source: Tomlinson Holman)