The Collaborative International Dictionary of English v.0.48:

Gravestone \Grave"stone\, n.
   A stone laid over, or erected near, a grave, usually with an
   inscription, to preserve the memory of the dead; a tombstone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gravestone
    n 1: a stone that is used to mark a grave [syn: gravestone,
         headstone, tombstone]