This is the name given to the branch of the Benedictine Order founded by Saint Bernard of Clairvaux (died 1090).
Saint Bernard was born near Dijon. He joined a monastery at Citeaux and then moved with his followers to establish another house at Clairvaux. This institution was enormously successful and a number of other houses were established in France, Britain and Ireland.
Bernard condemned the past abuses of the church and the luxuries, as he saw it, of other groups of the clergy. The Cistercians were an order of great austerity, wearing simple white habits.