Hinduism  is a religion that originated on the Indian subcontinent. With its foundations in the Vedic civilization, it has no known founder, being itself a conglomerate of diverse beliefs and traditions. It is considered the world's "oldest extant religion,"  and has approximately a billion adherents, of whom about 890 million live in India, placing it as the world's third largest religion after Christianity and Islam. Other countries with large Hindu populations include Nepal, Sri Lanka, Bangladesh, Indonesia and Malaysia.

Hinduism provides a vast body of scriptures. Divided as revealed and remembered, and developed over millennia, these scriptures expound an equally vast range of theology, philosophy, and myth, providing spiritual insights along with guidance on the practice of dharma (religious living). Among such texts, Hindus revere the Vedas along with the Upanishads as being among the foremost in authority, importance, and antiquity. The Bhagavad Gītā, a deeply profound conversation excerpted from the Mahābhārata, is widely seen as summarizing the spiritual teachings of the Vedas.

Etymology

The Persian term Hindu comes from the Sanskrit Sindhu, i.e. the Indus River. In the Rig Veda, the Indo-Aryans mention their land as Sapta Sindhu (the land of the seven rivers of the northwestern Indian subcontinent, one of them being the Indus). This corresponds to Hapta-Hendu in the Avesta (Vendidad: Fargard 1.18)—the sacred scripture of Zoroastrianism of Iran. The term was used for people who lived in the Indian subcontinent beyond the Sindhu.

From : en.wikipedia.org