The Collaborative International Dictionary of English v.0.48:

Myriameter \Myr"i*a*me`ter\, Myriametre \Myr"i*a*me`tre\, n. [F.
   myriam[`e]tre. See Myria-, and Meter.]
   A metric measure of length, containing ten thousand meters.
   It is equal to 6.2137 miles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

myriametre
    n 1: a metric unit of length equal to 10,000 meters [syn:
         myriameter, myriametre, mym]