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]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "headstone":
   arch, barrow, boundary stone, brass, bust, cairn, cenotaph, column,
   cornerstone, cromlech, cross, cup, cyclolith, dolmen, first stone,
   footstone, foundation stone, grave, gravestone, hoarstone,
   inscription, keystone, ledger, marker, mausoleum, megalith,
   memento, memorial, memorial arch, memorial column, memorial statue,
   memorial stone, menhir, monolith, monument, mound, necrology,
   obelisk, obituary, pillar, plaque, prize, pyramid, quoin,
   reliquary, remembrance, ribbon, rostral column, shaft, shrine,
   stela, stone, stupa, tablet, testimonial, tomb, tombstone, tope,
   trophy