TREFOIL (Lat. Irifolium, three-leaved plant, Fr. trefle, Ger. Dreiblall and Dreiblattbogen), the term in Gothic architecture given to the ornamental foliation or cusping introduced in the heads of window-lights, tracery, panellings, etc., in which the centre takes the form of a three-lobed leaf, one of the earliest examples being in the plate tracery at Winchester (1222-1235); see QUATREFOIL.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)