Kofun (古墳, from Sino-Japanese ancient grave) are megalithic tombs or tumuli in Japan, constructed between the early 3rd century and the early 7th century AD. The term Kofun is the origin of the name of the Kofun period, which indicates the middle 3rd century to early-middle 6th century. Many Kofun have distinctive keyhole-shaped mounds (zenpō-kōen fun (前方後円墳)), which are unique to ancient Japan.