A crusade is a holy war - that is, a war fought for God. It is also a pilgrimage. In the middle ages crusaders were usually known as 'pilgrims'; they took a pilgrim's vow and, like pilgrims, they expected to have their sins wiped out if they completed the journey. They also became known as 'crusaders' (croisés in French) because they received a cross (croix) to sew on to their clothes as a visible sign that they were going on crusade. As the concept of crusades developed, it was also considered that a crusade should be a just war: fought in a just cause and authorised by the appropriate earthly authority, in this case the pope.

Some historians believe that the only true crusades were those which went to the Holy Land. Many modern historians, however, believe that this is too narrow a definition. In the middle ages and early modern period, many military expeditions to Spain to fight the Muslims, to eastern Europe and the Baltic area to fight pagans, and even expeditions within Europe against religious or other enemies were called 'pilgrimages' and the warriors took the same vows and expected the same spiritual reward as if they went to the Holy Land.