Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Omphălus

（ὀμφαλός). Literally “the navel” (umbilicus), and applied to anything like the navel, round—e. g the knobs or bosses on a shield (see Clipeus), a knob on the yoke of a horse for fastening the reins, and, figuratively, the centre of anything. Thus Ogygia, the island of Calypso, is called ὀμφαλὸς θαλάσσης, and Delphi (q.v.) was also called ὀμφαλός, as being the supposed centre of the earth, from which, according to the myth, Zeus once despatched two eagles, one flying eastward and the other westward, with the result that they met again at Delphi. A marble boss marked the exact spot where they met.