Khat, Catha edulis, is an evergreen shrub of the highlands of Ethiopia, Kenya and Yemen whose young leaves and shoots are chewed fresh for a mild stimulant effect, comparable to strong coffee with a longer and more talkative arc.
Its religious dimension is Sufi, and the reasoning behind it is the same reasoning that made coffee acceptable in the same region at roughly the same time. The orders of Yemen and the Horn of Africa held long night devotions, dhikr, requiring sustained wakefulness and concentration, and khat made that possible. Tradition in Yemen credits Sufi shaykhs with its introduction, and the plant's spread from Ethiopia into Arabia is bound up with the orders.
The legal argument that followed is instructive. Khat, like coffee, was debated by jurists for centuries as to whether it fell under the prohibition of intoxicants, and the arguments turned on whether it clouds the intellect or sharpens it. Opinion divided and never fully settled, which is why khat is unremarkable in Yemen and Somalia and forbidden in Saudi Arabia.
Outside the devotional setting its use is overwhelmingly social and daily. Afternoon sessions in a dedicated room, with a specific etiquette and a recognised arc from talk to quiet, structure a great deal of ordinary Yemeni life, and the sessions are where business and disputes are conducted.