1. When referring to casting -- a sand unit placed inside of a mold so that when the metal is poured, the core will leave a hollow shape. 2. The magnetic center of a coil usually made of iron. 3. The primary part (engine block , alternator , starter , radiator , etc.) which has malfunctioned, but is still suitable for rebuilding or remanufacturing . You can exchange it for a new or rebuilt part. Thus, instead of paying full price for a new alternator, you can submit your old alternator as a core and pay a lower amount for the new alternator. "Core" is an abbreviation for "cash on return."