Real Dictionary

crusade
A crusade is a long and determined attempt to achieve something for a cause that you feel strongly about.
plural proper noun: The Crusades were the wars that were fought by Christians in Palestine against the Muslims during the eleventh, twelfth, and thirteenth centuries.
verb: If you crusade for a particular cause, you make a long and determined effort to achieve something for it.

crusade in American English
any of the military expeditions which Christians undertook from the 11th to the end of the 13th cent. to recover the Holy Land from the Muslims
to engage in a crusade
intransitive verb: to go on or engage in a crusade
noun: any of the military expeditions undertaken by the Christians of Europe in the 11th, 12th, and 13th centuries for the recovery of the Holy Land from the Muslims

crusade in British English
to campaign vigorously for something
noun: any of the military expeditions undertaken in the 11th, 12th, and 13th centuries by the Christian powers of Europe to recapture the Holy Land from the Muslims