Automatic Packet Reporting System (also known as "Automatic Position Reporting System")
APRS is best defined at a number of external sites:
Note: For the real APRS spec, there's a PDF "APRS101.pdf" (link?), plus two addendums on Bob Bruninga's web pages. The PDF was ratified by the major APRS authors at the time. Bob says Addendum 1 was ratified also. Addendum 2 has not been at this point (04/08/2016).