trefoil (trēˈfoil) [key] [O.Fr., = three-leaf], in botany, name for several plants, chiefly of the pulse family, having trifoliate leaves. Best known of the trefoils is clover. The bird's-foot trefoil ( Lotus corniculatus ) is an Old World forage plant and weed that has been naturalized in North America; the prairie trefoil ( L. americanus ) is a related native American plant. The shrubby trefoil is the hop tree. Tick trefoil is a name for the tickseed, or beggarweed.