An interceptor is a fighter aircraft optimized for combat with other aircraft. The term is roughly synonymous with air superiority fighter.
Interceptors tend to be optimized either for short-range point defense (e.g., MiG-21) or for long-range intercept of bombers and other strategic aircraft (e.g., MiG-25, F-14 Tomcat, MiG-31). In the U.S. Navy, the latter role is called fleet defense.
In general, pure interceptors have been deemphasized, in the west, by multirole fighters. Russia still seem interested in the long-range interceptor, and has been involved in joint development with India, especially for interceptors equipped with very long range missiles capable of killing high-value assets such as Airborne Warning and Control System aircraft.