The Collaborative International Dictionary of English v.0.48:

Chanson de geste \Chan`son" de geste"\ [F., prop., song of
   history.]
   Any Old French epic poem having for its subject events or
   exploits of early French history, real or legendary, and
   written originally in assonant verse of ten or twelve
   syllables. The most famous one is the Chanson de Roland.

         Langtoft had written in the ordinary measure of the
         later chansons de geste.                 --Saintsbury.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

chanson de geste
    n 1: Old French epic poems