Is called a crusade to any of the partly Christian military movements from Western Europe whose purpose was to place the Holy Land (the name by which Christians called Palestine) and the city of Jerusalem under the sovereignty of Christians. These movements extended between the 11th and 13th centuries, when Palestine was under the control of the Muslim Turks.
The rich and powerful knights of the Order of St. John of Jerusalem (Hospitallers) and the Knights Templar were created by the Crusades. The term is also used, by extension, to describe uncritically any religious war or even a political or moral movement.
Nine Crusades are traditionally spoken of, but in reality they were an almost permanent movement.