The Milion was a monument erected in the fourth century AD in Constantinople as a zero-mile marker - the starting point for the measurement of distances for all the roads leading to the cities of the Byzantine Empire per analogiam to Umbilicus urbis Romae.
See:
- Gilbert J. Gorski; James E. Packer The Roman Forum: A Reconstruction and Architectural Guide. Cambridge University Press, 2015, p. 155.