Webster's Revised Unabridged Dictionary · 1913

Trefoil

Trefoil , noun

[Latin tres three + English foil leaf; compare French frèfle, Italian trifoglio, Latin trifolium. See Tri-, Foil leaf, and compare Trifoly.]

1.
(Botany) Any plant of the genus Trifolium, which includes the white clover, red clover, etc.; -- less properly, applied also to the nonesuch, or black medic. See Clover, and Medic.
2.
(Architecture) An ornamental foliation consisting of three divisions, or foils.
3.
(Heraldry) A charge representing the clover leaf.