Source : Webster's Revised Unabridged Dictionary (1913)

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.] (Arch[ae]ol.)
   A monument consisting of three stones; especially, such a
   monument forming a kind of doorway, as among the ancient
   Celts.