Plain English Meaning
A code letter you write after your aircraft type on a flight plan to tell air traffic control that your aircraft can fly RNP routes accurately and has a working transponder that reports altitude.
Full Definition
An aircraft equipment suffix appended to the aircraft type in a flight plan that indicates the aircraft is equipped with Required Navigational Performance (RNP) capability along with transponder and Mode C altitude reporting. It tells ATC the aircraft can meet specific performance-based navigation accuracy requirements on RNP routes and procedures.