The short answer
A Supplemental Type Certificate (STC) is FAA approval for a modification to an aircraft, engine, or propeller that changes it from its original type design — for example an engine upgrade, avionics installation, or tip tanks. The STC is the approved data that makes such an alteration legal to install.
STCs in an aircraft's records
Each installed STC should appear in the aircraft's records, typically with a Form 337, the STC's paperwork, and any required flight-manual supplements. Popular airframes accumulate STCs over decades of ownership — engine conversions, gross-weight increases, autopilots, LED lighting.
For buyers and mechanics, the installed-STC list is part of the aircraft's true configuration: maintenance instructions, weight and balance, and even some inspection items follow the modified configuration rather than the original type design.
Related terms and reading
Keep every hour and inspection straight
Maggneto tracks engine times, inspections, and ADs from your actual logbooks — so terms like these become numbers you can act on. Browse the full maintenance glossary.