The Collaborative International Dictionary of English v.0.48:

Menhir \Men"hir\, n. [F. Armor. men stone + hir high.]
   A large stone set upright in olden times as a memorial or
   monument. Many, of unknown date, are found in Brittany and
   throughout Northern Europe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

menhir
    n 1: a tall upright megalith; found primarily in England and
         northern France [syn: menhir, standing stone]