Definition of "trefoil"

trefoil

/ˈtɹɛ.fɔɪl/

noun
plural trefoils
1. Any of several plants of the pea family, having compound, trifoliate leaves; especially one of the genus Trifolium.
2. A symbol having the shape of such leaves, especially when used as an architectural ornament.
  Quotations
  "The pristine, unbroken condition of the vessel – sometimes called a trefoil jug – caused the entire dig to 'come to a halt,' Søvsø remarked."
  2015 June 16, David DeMar, “Ancient Pitcher Discovered in Historic Danish City”, in New Historian
3. (topology) Ellipsis of trefoil knot.