Jizya was the poll tax levied under Muslim rule on free adult non-Muslim men of the protected communities, the ahl al-dhimma, in return for protection and for exemption from military service. Its scriptural warrant is Qur'an 9:29. In the jurists' formulations women, children, the indigent, the elderly, the disabled, the insane and in most schools monastics were exempt, and the tax was assessed in bands according to means.
What the classical jurists required and what actually happened are two different enquiries. The schools set differing schedules and differed over the manner of collection, some drawing from the phrase wa hum saghirun in 9:29 a requirement that payment express subordination, though the sense of that phrase is itself disputed among commentators, ancient and modern. The documentary record, in papyri, tax registers and administrative geographies, shows wide variation by period and province: rates changed, exemptions were granted and withdrawn, the poll tax was frequently confused or merged with the land tax kharaj, and early Umayyad governors sometimes resisted the fiscal consequences of conversion.
Two arguments contend over how to characterise the system. One holds that the dhimma granted legally guaranteed communal existence, internal religious jurisdiction and protection at a period when most polities offered religious minorities considerably less, and that the tax was the price of exemption from military obligation borne by others. The other holds that protection was conditional and explicitly hierarchical, that the accompanying restrictions on dress, worship, building and legal testimony were real wherever enforced, and that calling the arrangement tolerance measures it against the wrong standard. Both arguments are made by serious historians, and both draw on the same evidence; the disagreement is substantially about which comparison is the fair one.
The tax lapsed with the reorganisation of the Ottoman and other Muslim states in the nineteenth century and is levied by no modern state. It has been revived rhetorically, and in a small number of cases in fact, by insurgent movements: the Islamic State group imposed a payment described as jizya on Christians in Mosul in 2014, and its treatment of Yazidis, whom it did not recognise as a protected community at all, was found by a United Nations commission of inquiry to constitute genocide. Mainstream contemporary Muslim scholarship overwhelmingly treats the classical dhimma rules as historically conditioned and superseded by common citizenship.