An ancient district in what is now southwestern Oman, running from the sea up into the Dhofar Mountains and thence to the edge of the Rub` al-Khali. In the highlands are to be found scattered groves of the frankincense tree, source of what some regard as the finest of aromatic incenses. The region may also be roughly the locale where camels were first domesticated. 'Ad is known as the tribal Kingdom in which lay the city of Ubar, a major transshipment point for the frankincense trade in ancient times.