Quite frankly, it is anything that is not new, however, there is a wide range within the spectrum. Used equipment generally falls in at least one of the following categories:
Factory Re-Manned/Re-Conditioned - Good buy; usually comes with a 6-12 month warranty.
Overhauled - Good buy; usually comes with 6-12 month warranty; the term overhauled generally is attached to Instrumentation, Gyros, Servos, Motors, etc.; this term is sometimes applied to avionics which is somewhat misleading because circuitry can't really be overhauled just repaired or replaced; the term is used in conjunction with Radars that have had the Magnetron replaced or a Transponder that has had the Cavity replaced.
Yellow-Tagged (Form 8130) - Good buy; usually comes with no warranty to a 6 month warranty; the terms imply that the unit meets or exceeds all specifications as provided by the manufacturer.
Green-Tagged/Serviceable - Be careful; usually comes with no warranty; term simply means that the unit has not been tested but is expected to be repairable.
"As-Removed" - Buyer beware! Deals can be found here but as the term implies, it is in the same condition as it was when it was removed from the aircraft.
500-U Airport Road, Lititz, PA 17543 · 717-569-1953
The best choice with respect to condition and support is the first on the list. Within the remaining 4, there is also a wide variety of "repair" facilities who determine the equipment status. For example, the overhaul process of one shop may be superior to another. The less you know about from whom you are buying, the higher your risk factor. Balance your cost savings to your potential risk to determine the quality of the deal.