The Collaborative International Dictionary of English v.0.48:

Megalopolis \Meg`a*lop"o*lis\, Megapolis
\Meg`a"po*lis\(m[e^]g`[.a]*l[o^]p"[-o]*l[i^]s), n. [NL., fr. Gr.
   megalo`polis; me`gas, mega`lh, great + po`lis city.]
   1. A chief city; a very large cuity; a metropolis.
      [1913 Webster]

   2. A region in which the suburbs of several large cities
      merge to form one large predominantly urbanized area.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Megapolis \Me*gap"o*lis\, n. [NL., fr. Gr. me`gas great + po`lis
   city.]
   A metropolis. [Obs.] --Sir T. Herbert.
   [1913 Webster] Megarian