The Areopagus or Areios Pagos (Greek Ἄρειος Πᾶγος) is the 'Hill of Ares', north-west of the Acropolis, which in classical times functioned as the chief homicide court of Athens...In pre-classical times (before the 5th century BC), the Areopagus was the council of elders of the city, similar to the Roman Senate. Like the Senate, its membership was restricted to those who had held high public office, in this case that of Archon. In 462 BC, Ephialtes put through reforms which deprived the Areopagus of almost all its functions except that of a murder tribunal in favor of Heliaia.Thus, Paul's appearance before the Areopagus was not only a presentation before a group of philosophers and scholars, but actually a presentation to the leaders of the city of Athens. The equivalent of appearing before Congress... if Congress were also made up of highly educated philosophers.
In an unusual development, the Areopagus acquired a new function in the 4th century BC, investigating corruption...
(Source Wikipedia and ChristianAnswers.Net)