The Collaborative International Dictionary
Trilithons
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.
Wiktionary
trilithons
n. (plural of trilithon English)