The word "Hindu" came into vogue from the word "Sindhu" as the river at the north-western borders of India was called. While the land of the Indian sub-continent was called by the natives as Bharath, the Persians called the people living by and beyond the river after the name of the river, which was pronounced by them as "Hindu" and later as Indus by the Europeans and India by British. Thus by Hindu was denoted the people living in a particular geographical area irrespective of their religion. The word Hindu does not exist in the Vedas, though some latter scriptures have the word. Today the word Hindu has come to mean a particular religious culture. It is interesting that RSS (Rashtriya Swayamsevak Sangh), founded 85 years ago and considered to be India's largest Hindu organization, uses the word Hindu in its geographical context to denote Indian nationalism. However, VHP (Vishva Hindu Parishad), another large Hindu organization, uses the word Hindu to denote all religions born in India. There is also the word "Hindutva", used largely but not exclusively in politics, to denote the essence of Hinduism, which is stated by its proponents as the principle of "live and let live". This principle ensured that for many millennia the inhabitants of the Indian sub-continent did not need a word to identify themselves religiously, except for calling the religions themselves as elucidation of Sanatana Dharma or Eternal Law, because all religious expressions were accepted as the common heritage of everyone living in the ancient land of India. It was with the arrival of Islamic armies in India in the 8th century and more specifically the European traders in the 16th century that the word Hindu got to mean a religious group.
As to what makes a person Hindu in the religious sense, it is difficult to say because it has been famously said that what is true in Hinduism, its opposite is also true. Nevertheless three key words run through entire Hinduism - they are Dharma, Yoga and Karma. Anyone whose life has been touched by the understanding of at least these three words can be said to be a practising Hindu.
As to what makes a person Hindu in the religious sense, it is difficult to say because it has been famously said that what is true in Hinduism, its opposite is also true. Nevertheless three key words run through entire Hinduism - they are Dharma, Yoga and Karma. Anyone whose life has been touched by the understanding of at least these three words can be said to be a practising Hindu.