Source : Webster's Revised Unabridged Dictionary (1913)

Megalopolis \Meg`a*lop"o*lis\, n. [NL., fr. Gr. megalo`polis;
   me`gas, mega`lh, great + po`lis city.]
   A chief city; a metropolis. [R.]

Source : WordNet®

megalopolis
     n : a very large urban complex (usually involving several cities
         and towns)