name Turkmen first appears in written sources of the 10th century to distinguish those Oghuz groups who migrated south into the Seljuk domains and accepted Islam from those that had remained in the steppe. Gradually, the term took on the properties of an ethnonym and was used exclusively to designate Muslim Oghuz, especially those who migrated away from the Syrdariya Basin. By the 13th century, the term Turkmen supplanted the designation Oghuz altogether. The origin of the word Turkmen remains unclear. According to popular etymologies as old as the 11th century, the word derives from Turk plus the Iranian element manand, and means "resembling a Turk." Modern scholars, on the other hand, have proposed that the element man /men acts as an intensifier and have translated the word as "pure Turk" or "most Turk-like of the Turks." Seljuks In the 11th century, Seljuk domains stretched from the delta of the Amu Darya delta into Iran, Iraq, the Caucasus region, Syria, and Asia Minor. In 1040 the Seljuk Turks crossed the Oxus from the north, and having defeated Masud, sultan of Ghazni, raised Toghrul Beg, grandson of Seljuk, to the throne of Iran, founding the Seljukid dynasty, with its capital at Nishapur. A younger brother of Toghrul, Daud, took possession of Merv and Herat. Toghrul was succeeded by his nephew Alp Arslan (the Great Lion), who was buried at Merv. It was about this time that Merv reached the zenith of her glory. In 1055 Seljuk forces entered Baghdad, becoming masters of the Islamic heartlands and important patrons of Islamic institutions. Until these revolts, Turkmen tribesmen were an integral part of the Seljuk military forces. Turkmen migrated with their families and possessions on Seljuk campaigns into Azerbaijan and Anatolia, a process that began the Turkification of these areas. During this time, Turkmen also began to settle the area of present-day Turkmenistan. Prior to the Turkmen habitation, most of this desert had been uninhabited, while the more habitable areas along the Caspian Sea, Kopetdag Mountains, Amu Darya, and Murgap River (Murgap Deryasy) were populated predominantly by Iranians. The city-state of Merv was an especially large sedentary and agricultural area, important as both a regional economic-cultural center and a transit hub on the Silk Road. The last powerful Seljuk ruler, Sultan Sanjar (d. 1157), witnessed the fragmentation and destruction of the empire because of attacks by Turkmen and other tribes. During the reign