(usu in pl) an unusual visual or sound acoustic effect; esp one introduced into a film or prerecorded television production by special processing
Special effects are those not possible in mere photography or sound recording. They included matte work, split screens, models, stop motion, and so on. They were the work of specialists (like Willis O'Brien on King Kong), but in the 1980s, with the computer and digitalization, special effects has come to mean the electronic semblance of photography with ‘impossible’ subjects (e.g. in Jurassic Park) — David Thomson