Chalcedon (kăl`sĭdŏn, –dən, kălsē`dən), ancient Greek city of Asia Minor, on the Bosporus. It was founded by Megara on the shore opposite Byzantium in 685 B.C. Taken by the Persians and recovered by the Greeks, it was later a possession of the kings of Bithynia, from whom it passed (A.D. 74) to Rome. The Council of Chalcedon was held there in A.D. 451. The site is in the suburbs of İstanbul.