Kabul was known by different names throughout history. Its meaning is unknown, but the "Concise Oxford Dictionary of World Place Names" notes that it "certainly pre-dates the advent of Islam when it was an important centre on the route between India and the Hellenic world". In Sanskrit, it was known as "Kubha", whereas Greek authors of classical antiquity referred to it as "Kophen", "Kophes" or "Koa". The Chinese traveler Xuanzang (fl. 7th century CE) recorded the city as "Kaofu". The name "Kabul" was first applied to the Kabul river before being applied to the area situated between the Hindu Kush and Sindh (present-day Pakistan). This area was also known as Kabulistan. Alexander Cunningham (died 1893) noted in the 19th century that "Kaofu" as recorded by the Chinese was in all likelihood the name of "one of the five Yuchi or Tukhari tribes". Cunningam added that this tribe gave its name to the city after it was occupied by them in the 2nd century BCE. According to the "Oxford Encyclopedia of the Modern World", this "supposition seems likely" as the Afghan historian Mir Ghulam Mohammad Ghobar (1898–1978) wrote that in the Avesta (sacred book of Zoroastrianism), Kabul was known as "Vaekereta", whereas the Greeks of antiquity referred to it as "Ortospana" ("High Place"), which corresponds to the Sanskrit word "Urddhastana", which was applied to Kabul. The Greek geographer Ptolemy (died CE) recorded Kabul as "Kabura".