The Collaborative International Dictionary of English v.0.48:

Stonehenge \Stone"henge\, n.
   An assemblage of upright stones with others placed
   horizontally on their tops, on Salisbury Plain, England, --
   generally supposed to be the remains of an ancient Druidical
   temple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Stonehenge
    n 1: an ancient megalithic monument in southern England;
         probably used for ritual purposes