The Collaborative International Dictionary of English v.0.48:

Headstone \Head"stone`\ (-st[=o]n`), n.
   1. The principal stone in a foundation; the chief or corner
      stone. --Ps. cxviii. 22.
      [1913 Webster]

   2. The stone at the head of a grave.
      [1913 Webster]

WordNet (r) 3.0 (2006):

headstone
    n 1: the central building block at the top of an arch or vault
         [syn: keystone, key, headstone]
    2: a stone that is used to mark a grave [syn: gravestone,
       headstone, tombstone]