Still an ambiguous notion, the word “crusade” wasn’t used when they began; it only came into use in its current meaning in the late 18th century. The root of the word comes from “croisade,” which was first used in the 16th century, and is itself derived from the Latin word “cruciata,” meaning to mark with a cross.
The original crusades were instead referred to as “iter” or “peregrinatio” meaning journey and pilgrimage, respectively.
Crusades had to be sanctioned by the pope, and they were officially used to conduct battle on groups deemed to be enemies of Christendom.
Crusaders were often promised absolution from purgatory in the afterlife. They were also usually landless nobles or peasants who were in search of creating a future for themselves by seizing land from their Muslim enemies.
In the year 1095, Pope Urban II proclaimed the First Crusade. The ideology adopted by crusaders was that Christianity was a universal salvation that should be adopted by all. If people did not convert, then they could be murdered in the name of God. The crusades grew out of the early Christian tradition of pilgrimages.