Bernard Lewis says Jihad is an unlimited offensive to bring the whole world under Islamic law; Christian crusades a defensive, limited response to, and imitation of, jihad

From pp.233-234 of The Middle East: A Brief History of the Last 2000 Years:

Even the Christian crusade, often compared with the Muslim jihad, was itself a delayed and limited response to the jihad and in part also an imitation. But unlike the jihad it was concerned primarily with the defense or reconquest of threatened or lost Christian territory...The Muslim jihad, in contrast, was perceived [by Muslims] as unlimited, as a religious obligation that would continue until all the world had either adopted the Muslim faith or submitted to Muslim rule.… The object of jihad is to bring the whole world under Islamic law.