The word Hindū is derived from Indo-Aryan/Sanskrit root Sindhu. The Proto-Iranian sound change *s > h occurred between 850–600 BCE, according to Asko Parpola.
The use of the English term "Hinduism" to describe a collection of practices and beliefs is a recent construction: it was first used by Raja Ram Mohun Roy in 1816-17. The term "Hinduism" was coined in opposition to other religions. Before the British began to categorise communities strictly by religion, Indians generally did not define themselves exclusively through their religious beliefs; identities were segmented on the basis of locality, language, caste, occupation and sect.
The word "Hindu" is much older, and it is believed that it was used as the name for the Indus River in the northwestern part of the Indian subcontinent (modern day Pakistan and Northern India).[note 7] According to Gavin Flood, "The actual term Hindu first occurs as a Persian geographical term for the people who lived beyond the river Indus (Sanskrit: Sindhu)", more specifically in the 6th-century BCE inscription of Darius I (550–486 BCE). The term Hindu in these ancient records is a geographical term and did not refer to a religion. Among the earliest known records of 'Hindu' with connotations of religion may be in the 7th-century CE Chinese text Record of the Western Regions by Xuanzang, and 14th-century Persian text Futuhu's-salatin by 'Abd al-Malik Isami.[note 8]
Thapar states that the word Hindu is found as heptahindu in Avesta – equivalent to Rigvedic sapta sindhu, while hndstn (pronounced Hindustan) is found in a Sasanian inscription from the 3rd century CE, both of which refer to parts of northwestern South Asia. The Arabic term al-Hind referred to the people who live across the River Indus. This Arabic term was itself taken from the pre-Islamic Persian term Hindū, which refers to all Indians. By the 13th century, Hindustan emerged as a popular alternative name of India, meaning the "land of Hindus".[note 9]
The term Hindu was later used occasionally in some Sanskrit texts such as the later Rajataranginis of Kashmir (Hinduka, c. 1450) and some 16th- to 18th-century Bengali Gaudiya Vaishnava texts including Chaitanya Charitamrita and Chaitanya Bhagavata. These texts used it to distinguish Hindus from Muslims who are called Yavanas (foreigners) or Mlecchas (barbarians), with the 16th-century Chaitanya Charitamrita text and the 17th-century Bhakta Mala text using the phrase "Hindu dharma". It was only towards the end of the 18th century that European merchants and colonists began to refer to the followers of Indian religions collectively as Hindus.
The term Hinduism, then spelled Hindooism, was introduced into the English language in the 18th century to denote the religious, philosophical, and cultural traditions native to India.