An omphalos is an ancient religious stone artifact, or baetylus. In Greek, the word omphalos means "navel" (compare the name of Queen Omphale). According to the ancient Greeks, Zeus sent out two eagles to fly across the world to meet at its center, the "navel" of the world. Omphalos stones used to denote this point were erected in several areas surrounding the Mediterranean Sea; the most famous of those was at the oracle in Delphi.
Sources: http://en.wikipedia.org/wiki/Omphalos
Umbilicus, hump. Ovoid resp. beehive shaped stone mark of uncertain meaning, sometimes depicted with attributes of birds and covered with reticular basketwork. The best-known exemplar is the Delphic Omphalus, as oracle seat at first of Gaia, then belonging to Apollon; according to Ninion-Pinax represented also in the sanctuary of the Mother of Earth of Eleusis. The Delphic Omphalos today no more is regarded as meteoric object of the aniconic stone cult (Baetyl) but as prehellenic sacrifice mark of the earth-goddess as synthesis of Tymbos (= doomed tomb) and altar (for chthonic blood pouring). In contrast the imaginations of the Omphalos as an umbilicus of the earth are secondary. Attempts to integrate it into orientalic religious relations are questionable; connections to Aegaean, Near Eastern or Celtic stone entities, and interpretations in the context of fertility symbolism remains controversial. -- Der kleine Pauly