Definition of "trilithon"

[…] thus the two opposite trilithons which are nearest to the north-west or grand entrance, were 19 feet in height, the two next opposite trilithons correspond in the height of 20 feet 3 inches, and the trilithon at the back of the stone of astronomic observation, or as erringly called the Altar-stone, raised its ascent to the astonishing height of 25 feet.

1846, Edward Duke, The Druidical Temples of the County of Wilts, page 158

The two trilithons forming the wall on each side of the cella therefore pass on the mutual neighbourhood to each other, despite the fact that they are set a slight distance apart. But owing to the oblique placing of the pair of trilithons nearest the head-end the spread is greatly reduced on that side; the head-trilithon stands at the exact spot where it ceases.

1983, Hans van der Laan, Architectonic Space: Fifteen Lessons on the Disposition of the Human Habitat, page 193

Within the sarsen circle are the five impressive trilithons, forming a horseshoe-shaped ring of huge stones.

2007, Brad Olsen, Sacred Places Europe: 108 Destinations, Consortium of Collective Consciousness, page 45

[…] whereas the four-square gallows would have been more likely to suggest to his contemporary Blake, who normally used symbolism for its own sake, a resemblance to one of the four-square trilitha at Stonehenge.

2009, John Beer, Romanticism, Revolution and Language: The Fate of the Word from Samuel Johnson to George Eliot, Cambridge University Press, page 84