Trefoil — Chambers's Encyclopaedia

Trefoil, a name given to many herbaceous plants with leaves of three leaflets, as Clover (q.v.), Lotus (q.v.), Medick (q.v.), Buckbean (q.v.), &c. The term is used also in Architecture (for a three-lobed aperture in tracery) and in Heraldry.

A black and white illustration of a trefoil, which is a three-lobed aperture or trefoil design, often used in architecture or heraldry. — Trefoil in Architecture.