Trefoil

Trefoil (from Latin trifolium, three-leaved plant, French trèfle, German Dreiblatt and Dreiblattbogen) is a term in Gothic architecture given to the ornamental foliation or cusping introduced in the heads of window-lights, tracery, panellings, etc., in which the center takes the form of a three-lobed leaf, one of the earliest examples being in the plate tracery at Winchester (1222 - 1235); see Quatrefoil .

Trefoil sign indicating radioactivity

Trefoil is also:

• a common name for some kinds of clover, which have three-foiled leaves.
• In mathematical knot theory, a trefoil refers to a trefoil knot.
• the name of the symbol used to indicate radioactivity
• a variation of the solitaire card game La Belle Lucie.