omphalos (n.)

also omphalus, "sacred stone," 1850, from Greek omphalos, literally "navel," later also "hub" (as the central point), from PIE *ombh-alo-, from root *nobh-/*ombh- "navel" (see navel). Especially as the name of the rounded or conical stone in the shrine at Delphi, regarded by the ancients as the center of the world. Related: Omphalic.