The Collaborative International Dictionary of English v.0.48:

Megameter \Meg"a*me`ter\, Megametre \Meg"a*me`tre\, n. [Mega- +
   meter, metre, n., 2.]
   In the metric system, one million meters, or one thousand
   kilometers.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Megameter \Me*gam"e*ter\, n. [Mega- + -meter: cf. F.
   m['e]gam[`e]tre.] (Physics)
   1. An instrument for determining longitude by observation of
      the stars.
      [1913 Webster]

   2. A micrometer. [R. & Obs.] --Knight.
      [1913 Webster] Megameter