A set of aircraft and flight crew requirements needed to support performance-based navigation operations within a defined airspace. There are two kinds of navigation specifications:
Required navigation performance (RNP) specification. A navigation specification based on area navigation that includes the requirement for performance monitoring and alerting, designated by the prefix RNP, e.g. RNP 4, RNP APCH.
Area navigation (RNAV) specification. A navigation specification based on area navigation that does not include the requirement for performance monitoring and alerting, designated by the prefix RNAV, e.g. RNAV 5, RNAV 1.
ICAO (2006) Official definition, Annex 10, Vol. I, Aeronautical Telecommunications — Radio Navigation Aids
The term RNP, previously defined as “a statement of the navigation performance necessary for operation within a defined airspace”, has been removed (...) as the concept of RNP has been overtaken by the concept of PBN. The term RNP (...) is now solely used in the context of navigation specifications that require performance monitoring and alerting, e.g. RNP 4 refers to the aircraft and operating requirements, including a 4 NM lateral performance with on-board performance monitoring and alerting that are detailed in The PNB Manual. (same source)