The first time the name Hindu occurs is at the time of the Persian King Darius the great (550 B.C). It is referred to as Hidush. The mountain range of Hindu Kush denoted the boundary of the Hindu people. The Chinese traveller Hiuen Tsiang also gives a name which is the Chinese equivalent of Hindu. The Arabs got the name from the Persians. The close connection between Persia and India dating back to the Vedic period has not been researched fully. The common factors of Ahura Mazda and our scriptures have been pointed out by some scholars. The boundary of the Hindus was the Hindu Kush mountains and not the Sindhu river. Gandhara and Khamboja kingdoms were beyond the Sindhu river. According to Nigel Allan, a historian, there were at least two meanings for "Hindu Kush" common centuries ago "mountains of India" and "sparkling snows of India" - he notes that the name is clearly applied from a Central Asian perspective. Others maintain that the name Hindu Kush is probably a corruption of Hindi-Kash or Hindi-Kesh, the boundary of Hind. Sanatana Dharma is a word which was taken from a medieval copper plate. I have given the history of its origin in one of my posts. I could post again if members are interested. The word is Sanatana Dharma in Sanskrit. Not Sanatan dharma as it is said in Hindi.