An obsolete word, now supplanted by crusade, from the french croiser, to grow (also the root of croissant, a crescent, or crescent-shaped pastry): a crusader's campaign would be initiated in order to grow the territory or influence of an existing power, such as say, the church, the slave trade, or Belgium.

Warning: only parts of this are true. Webster 1913 is right. Go to Crusade.