/tree"foyl, tref"oyl/ , n.
1. any of numerous plants belonging to the genus Trifolium, of the legume family, having usually digitate leaves of three leaflets and reddish, purple, yellow, or white flower heads, comprising the common clovers.
2. any of various similar plants.
3. Archit. an ornament composed of three lobes, divided by cusps, radiating from a common center.
4. such an ornamental figure used by the Girl Scouts as its official emblem.
adj.
5. of, pertaining to, or shaped like a trefoil.
[ 1350-1400; ME trifoil trifolium triple leaf, the three-leaved plant, clover, equiv. to tri- TRI- + folium leaf ]