The word quatrefoil etymologically means "four leaves", and applies to general four-lobed shapes in various contexts.
In architecture and traditional Christian symbolism, a quatrefoil is a symmetrical shape which forms the overall outline of four partially-overlapping circles of the same diameter.
The quatrefoil enjoyed its peak popularity during the Gothic Revival and Renaissance, but can still be seen on countless churches and cathedrals today. It is most commonly found as tracery, mainly in Gothic architecture, where a quatrefoil can often be seen at the top of a Gothic arch, sometimes with stained glass on the interior.
In art, the quatrefoil is a type of decorative framework (mainly used in engraving), consisting of an architectural quatrefoil combined with a square (just as the trefoil is often combined with an equilateral triangle). Among the most famous works of art employing the quatrefoil are bronze panels on the North Doors of the Baptistery in Florence by Lorenzo Ghiberti and "Head of an Angel" by Piero della Francesca.
The four-leaf clover is a symbol of good luck.