Plain English Meaning
An instrument approach that is built to be flown using GPS only. The pilot does not need to tune or back up the procedure with a ground-based navigation station. GPS handles every part of it, from joining the approach to flying the missed approach.
Full Definition
Instrument approach procedures that are designed and titled specifically for use with GPS as the primary navigation source, independent of any ground-based navigation aid such as a VOR, NDB, or localizer. The approach chart is titled with 'GPS' or 'RNAV (GPS)' in the procedure name, and the entire approach -- from the initial approach fix through the missed approach -- is flown using GPS guidance.