Source : Webster's Revised Unabridged Dictionary (1913)

Acropolis \A*crop"o*lis\, n. [Gr. 'akro`polis; 'a`kros extreme +
   po`lis city.]
   The upper part, or the citadel, of a Grecian city;
   especially, the citadel of Athens.

Source : WordNet®

acropolis
     n : the citadel in ancient Greek towns