(“struggle,” or “battle”), a religious duty imposed on Muslims to spread Islam by waging war; jihad has come to denote any conflict waged for principle or belief and is often translated to mean “holy war.”

Islam distinguishes four ways by which the duty of jihad can be fulfilled: by the heart, the tongue, the hand, and the sword. The first consists in a spiritual purification of one's own heart by doing battle with the devil and overcoming his inducements to evil. The propagation of Islam through the tongue and hand is accomplished in large measure by supporting what is right and correcting what is wrong. The fourth way to fulfill one's duty is to wage war physically against unbelievers and enemies of the Islamic faith. Those who professed belief in a divine revelation—Christians and Jews in particular—were given special consideration. They could either embrace Islam or at least submit themselves to Islamic rule and pay a poll and land tax. If both options were rejected, jihad was declared.

Modern Islam places special emphasis on waging war with one's inner self. It sanctions war with other nations only as a defensive measure when the faith is in danger.

Throughout Islamic history, wars against non-Muslims, even though with political overtones, were termed jihads to reflect their religious flavour. This was especially true in the 18th and 19th centuries in Muslim Africa south of Sahara, where religiopolitical conquests were seen as jihads, most notably the jihad of Usman dan Fodio, which established the Sokoto caliphate (1804) in what is now northern Nigeria. The Afghan War in the late 20th and early 21st centuries was also viewed by many of its participants as a jihad, first against the Soviet Union and Afghanistan's Marxist government and, later, against the United States. During that time, Islamic extremists used the theory of jihad to justify violent attacks against Muslims whom the extremists accused of apostasy (Arabic riddah).