Webster's English Dictionary

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trefoil

tre.foil \'tre--.fo.il, 'tref-.o.il\ n [ME, fr. MF trefeuil, fr. L 
   trifolium, fr. tri- + foliumX leaf - more at BLADE 1a: CLOVER; broadly : 
   any of several trifoliolate leguminous  herbs 1b: a trifoliolate leaf  2: 
   an ornament or symbol in the form of a stylized trifoliolate leaf