A monument consisting of three stones; especially, such a monument forming a kind of doorway, as among the ancient Celts.

A stone arch made from 2 upright slabs of rock or wood, with one lying atop. Commonly used as an altar.

an arrangement of three stones, two uprights with a lintel placed at their top in a kind of inverted "U" shape

A structure made up of three stones: two uprights and a horizontal lintel. Three trilithons stand within the sarsen horseshoe at Stonehenge

(6) -- a construction with two upright posts and a horizontal one set on top of them; often the lintel slab set over the doorway of a tholos tomb or over those of Early and Middle Minoan round tombs (Warren, 148)

A stone arch made from two upright slabs with one lying atop these. Trilithons are featured in Stonehenge as well as the circle visualization in The Standing Stones Book of Shadows.