1. Any of the military expeditions undertaken by European Christians in the 11th, 12th, and 13th centuries to recover the Holy Land from the Muslims: Mrs. Hathaway, Sandy's history teacher, told her students about the religious crusades in medieval times.
2. A holy war undertaken with papal sanction: Formally, a crusade was a religious war that was embarked on with the pope's encouragement and permission.
3. A strong movement for a cause or against an abuse: The town decided to organize a crusade against drunk driving.

The Crusades refers to the nine historical European-Christian military expeditions to the Muslim-controlled Holy Land. The term "crusaders" refers or those taking part in those campaigns.

In English, the term crusade has come to refer to any type of campaign that is conducted with an emphasis on zeal and persistence. Even in these days it may refer to a contemporary attack against Islam, whether perceived or real, asserted to be of similar nature or basis as the historic Crusades.

To campaign against something or to promote something vigorously: After recovering from his addiction to alcohol, he crusaded against the use of alcohol as a drug because he considered it to be wrong.