There are different definitions. But in the sense of the Christian crusades, it mostly refers to the recovery of land from Muslim invaders. Personally I think a thorough crusade would be full conquest, which means he conquered must convert.
If you say Christians didn’t do that, you’d be right — which is why dhimmitude is in the future for most Christians.
ISIS, however, is conquering and forcing conversions in the lands they are taking. You must convert or die (if you’re lucky — many are not given the chance to live). If you live in these conquered Muslim lands as a non-Muslim, you must live in dhimmitude. Third-class citizen status.
You may be thinking of a papal “crusade”.
noun
1.
(often initial capital letter) 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.
2.
any war carried on under papal sanction.
3.
any vigorous, aggressive movement for the defense or advancement of an idea, cause, etc.:
You are right - I was 100% thinking of a papal crusade.