The Collaborative International Dictionary of English v.0.48:

Decameter \Dec"a*me`ter\, Decametre \Dec"a*me`tre\, n. [F.
   d['e]cam[`e]tre; Gr. de`ka ten + m[`e]tre. See Meter.]
   A measure of length in the metric system; ten meters, equal
   to about 393.7 inches.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decameter
    n 1: a metric unit of length equal to ten meters [syn:
         decameter, dekameter, decametre, dekametre, dam,
         dkm]