Areopagus was a council that ruled ancient Athens from 600s to the 400s B.C. Its removal from power by Ephialtes in 462-61 B.C. was the beginning of the Athenian democracy.
The council had the name "Areopagus" because it met on a hill having that name in Athens.
In the New Testament Book of Acts 17:22, where Saint Paul stands up and addresses the men of Athens, the Greek Ἀρείου Πάγου Areiou Pagou "Ares Hill", has been variously translated as "Mars' Hill" and "Areopagus".