The Collaborative International Dictionary
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
n. A structure consisting of two stone pillars supporting a horizontal stone
Wikipedia
A trilithon (or trilith) is a structure consisting of two large vertical stones (posts) supporting a third stone set horizontally across the top (lintel). It is commonly used in the context of megalithic monuments. The most famous trilithons are those of Stonehenge in England, those found in the Megalithic temples of Malta, both of which are UNESCO World Heritage Sites, and the Osirion in Egypt.
The word trilithon is derived from the Greek "having three stones" (τρι-/tri- 'three' + λίθος/lithos 'stone') and was first used by William Stukeley.
The term also describes the groups of three stones in the Hunebed tombs of the Netherlands and the three massive stones forming part of the wall of the Roman Temple of Jupiter at Baalbek, Lebanon. Far from Europe and the Middle East, another famous trilithon is the Haʻamonga ʻa Maui in Tonga, Polynesia. __NOTOC__
Usage examples of "trilithon".
He nodded toward the angular, insectile shadow in the darkness, more monsterlike than ever in his ring of electronic trilithons.
In the grey-rinsed light, rising from the rain-dimpled water, the jungle gyms and the seesaws and the elaborate swing sets appeared strange to Joe, not at all like what they were, but like a steel-pipe Stonehenge more mysterious even than the ancient rock megaliths and trilithons on England's Salisbury Plain.