In botany, 'trefoil' is an alternative name for clover. In architecture, a trefoil is an ornament composed of three lobes, divided by cusps, radiating from a common centre. (A cusp is a pair of curves, tangent to the real or imaginary line defining the area decorated, and meeting at a point within the area.)
Dictionary.com gives "such a figure used by the Girl Scouts as its official emblem" as one definition for trefoil.
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