The Collaborative International Dictionary of English v.0.48:

graviton \graviton\ n. (Physics)
   A theoretical gauge boson that mediates the (extremely weak)
   gravitational interactions between particles; the quantum of
   the gravitational field, having zero rest mass and a spin of
   2.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

graviton
    n 1: a gauge boson that mediates the (extremely weak)
         gravitational interactions between particles