The Collaborative International Dictionary of English v.0.48:

Areopagus \Ar`e*op"a*gus\, n. [L., fr. Gr. ?, and ? ?, hill of
   Ares (Mars' Hill).]
   The highest judicial court at Athens. Its sessions were held
   on Mars' Hill. Hence, any high court or tribunal.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Areopagus
    n 1: a hill to the to the west of the Athenian acropolis where
         met the highest governmental council of ancient Athens and
         later a judicial court
    2: the highest governmental assembly in ancient Athens (later a
       judicial court)

Easton's 1897 Bible Dictionary:

Areopagus
   the Latin form of the Greek word rendered "Mars' hill." But it
   denotes also the council or court of justice which met in the
   open air on the hill. It was a rocky height to the west of the
   Acropolis at Athens, on the south-east summit of which the
   council was held which was constituted by Solon, and consisted
   of nine archons or chief magistrates who were then in office,
   and the ex-archons of blameless life.

     On this hill of Mars (Gr. Ares) Paul delivered his memorable
   address to the "men of Athens" (Acts 17:22-31).


Hitchcock's Bible Names Dictionary (late 1800's):

Areopagus, the hill of Mars


Bouvier's Law Dictionary, Revised 6th Ed (1856):

AREOPAGUS. A tribunal established in ancient Athens, bore this name. It is
variously represented; some considered as having been a model of justice and
perfection, while others look upon it as an aristocratic court, which had a
very extended jurisdiction over all crimes and offences, and which exercised
an absolute power. See Acts 17, 19 and 22.