Source : Webster's Revised Unabridged Dictionary (1913)

Headstone \Head"stone`\, n.
   1. The principal stone in a foundation; the chief or corner
      stone. --Ps. cxviii. 22.

   2. The stone at the head of a grave.

Source : WordNet®

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