A labyrinth is unicursal with a well-defined path that leads to the center and back out again (as opposed to a maze which offers a choice of paths including dead ends and cul-de-sacs). Found in almost every religious tradition around the world, labyrinths have been known to the human race for over four thousand years. The oldest surviving labyrinth dates from 2500-2000 BCE.
The labyrinth concept developed significantly in the Middle Ages and found its way into European Christian cathedrals, one of the most famous being at Chartres Cathedral. During medieval times, those who were unable to make actual pilgrimages to sacred sites walked the Chartres labyrinth as a pilgrimage of the soul. Grace Cathedral in San Francisco, having first presented their labyrinth in 1991, played a large part in sparking the current resurgance of this ancient spiritual tool.