It means "justifiable war". The debatable part of the justification, of which many are possible. No one wants to claim that they started a war in order to rape, pillage and burn, since these are activities frowned upon by all the Sand People religions, so they invent excuses. But a Crusade originally referred to a specific act of aggression with a specific purpose in a specific place, and the word jihad did not mean that. A crusade always refers to some sort of preemptive action, never a defensive one.
Al Qaeda is defending a past way of life that is increasingly threatened economically. socially, politically and morally, and is certainly doomed in the long run. Technology is rendering it obsolete and will continue to do so. The same is happening with the fundie Christians. They are doomed as well. Islam will survive, just as Christianity will survive, but the fanatical element is doomed in both cases.
Both words are used metaphorically, but jihad can refer to an act of defense, while the word crusade always refers to some sort of attack.
The Crusades were not a noble struggle in reality. Only those who are ignorant of the various Crusades would call it noble in any way. Establishing the County of Edessa was not a noble venture, just an act of unjustifiable aggression. Jerusalem was not improved by the Crusaders, nor did putting the Holy Land in Christian hands result in Jesus gratefully returning to claim the throne as suggested in the Book of Revelation.
The actual Crusades were a miserable and bloody flop. I expect that Al Qaeda's jihad will be a less prolonged, and less justifiable flop.
