Plain English Meaning
A satellite-based navigation unit that the FAA has cleared for use when flying on instruments. Not every GPS in an aircraft qualifies — it must meet specific standards and be officially installed and documented for IFR flight.
Full Definition
A Global Navigation Satellite System receiver that meets the FAA's technical and operational standards (TSO-C129, TSO-C145, or TSO-C146) for use as a primary navigation source under Instrument Flight Rules. To be IFR approved, the installation must be certified for the specific (, , or ), be properly installed per an package, and be listed in the aircraft's or Supplement as for IFR use.