The Collaborative International Dictionary of English v.0.48:

Trilithon \Tril"i*thon\ (tr[i^]l"[i^]*th[o^]n), n.; pl.
   Trilithons. [NL., fr. Gr.? of or with three stones; ? (see
   Tri-) + ? stone.] (Archaeol.)
   A monument consisting of three stones; especially, such a
   monument forming a kind of doorway, as among the ancient
   Celts.
   [1913 Webster]