Толковый словарь английского языка Oxford English Reference - trefoil

n. & adj. --n. 1 any leguminous plant of the genus Trifolium, with leaves of three leaflets and flowers of various colours, esp. clover. 2 any plant with similar leaves. 3 a three-lobed ornamentation, esp. in tracery windows. 4 a thing arranged in or with three lobes. --adj. of or concerning a three-lobed plant, window tracery, etc. Derivatives trefoiled adj. (also in comb.). Etymology: ME f. AF trifoil f. L trifolium (as TRI-, folium leaf)