Plain English Meaning
It is the FAA rulebook that says what a WAAS-capable GPS unit must be able to do before it can be installed in an aircraft and used on its own for navigation, including instrument approaches.
Full Definition
An FAA Technical Standard Order that sets the minimum performance standards for stand-alone GPS/WAAS navigation equipment installed in civil aircraft. Equipment built to TSO-C146A is approved for use as a primary means of navigation in oceanic, domestic en route, terminal, and approach phases of flight, including LNAV, LNAV/VNAV, and .