The Collaborative International Dictionary of English v.0.48:

Cromlech \Crom"lech\ (kr[o^]m"l[e^]k), n. [W. cromlech; crom
   bending or bent, concave + llech a flat stone; akin to Ir.
   cromleac.] (Arch[ae]ol.)
   A monument of rough stones composed of one or more large ones
   supported in a horizontal position upon others. They are
   found chiefly in countries inhabited by the ancient Celts,
   and are of a period anterior to the introduction of
   Christianity into these countries.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cromlech
    n 1: a prehistoric megalithic tomb typically having two large
         upright stones and a capstone [syn: dolmen, cromlech,
         portal tomb]