In general terms, the omphalos is the center or the hub of everything. An omphalos can be positive or negative, as in the ompahlos of confusion or the omphalos of joy, but the most common usage takes us back to ancient Greece.
In Greek history terms, the omphalos is the rounded stone, like the one at Delphi, that represents the navel of the world. The myth goes that Zeus, who wanted to locate the center of the earth, set two eagles at either end of the world. Like a poorly constructed word problem: If the eagles flew at the same speed, starting simultaneously, the point at which their paths cross must be the center of the earth. Where is it?
Delphi, of course! So, that's where Zeus placed the omphalos, the navel stone!
To read more about the mythology surrounding the omphalos, check out this brief and accessible article by an archeology student .