tre·foil 
(trēfoil′, trĕfoil′)
(trēfoil′, trĕfoil′)n.
1. Any of various plants of the pea family, chiefly of the genera Trifolium and Lotus, having compound trifoliolate leaves.
2. An ornament, symbol, or architectural form having the appearance of a trifoliolate leaf.
[Middle English, from Anglo-Norman trifoil, from Latin trifolium : tri-, tri- + folium, leaf; see bhel-3 in the Appendix of Indo-European roots.]
trefoil