The Areopagus or Areios Pagos (Ancient Greek: Ἄρειος Πάγος) is the "Rock of Ares", north-west of the Acropolis, which in classical times functioned as the high Court of Appeal for criminal and civil cases in Athens. Ares was supposed to have been tried here by the gods for the murder of Poseidon's son Alirrothios (a typical example of an aetiological myth). 

The origin of its name is not clear. In Greek, pagos means big piece of rock. Areios could have come from Ares or from the Erinyes, as on its foot was erected a temple dedicated to the Erinyes where murderers used to find shelter so as not to face the consequences of their actions. Later, the Romans referred to the rocky hill as "Mars Hill", after Mars, the Roman God of War. Near the Areopagus was also constructed the basilica of Dionysius Areopagites.  

http://en.wikipedia.org/wiki/Areopagus