The Collaborative International Dictionary of English v.0.48:

Tombstone \Tomb"stone`\ (t[=oo]m"st[=o]n`), n.
   A stone or small stone monument erected over a grave, bearing
   the name of the dead person interred there, to preserve the
   memory of the deceased.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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