"period of four years" (between Olympic games), late 14c., from Old French olimpiade "period of four years," from Latin Olympiadem, from Greek olympiados, genitive of Olympias "Olympian, of or pertaining to Olympus," an epithet of the muses, as a noun, "the Olympic games; a victor at Olympia; the space of four years between the celebrations of the Olympic games"(see Olympic). Used by ancient Greeks as a unit in computing time. Revived in modern usage with revival of the games, 1896. Related: Olympiadic.
To turn an Olympiad into a year B.C, multiply by 4, add the year of the Olympiad less 1, and subtract from 780. [Century Dictionary]