Devices used for influencing tooth position. Orthodontic appliances may be classified as fixed or removable, active or retaining, and intraoral or extraoral. (Boucher's Clinical Dental Terminology, 4th ed, p19)
The phase of orthodontics concerned with the correction of malocclusion with proper appliances and prevention of its sequelae (Jablonski's Illus. Dictionary of Dentistry).