In modern Indian languages, 'astika' and 'nastika' generally mean 'theist' and 'atheist', respectively. But in Sanskrit philosophical literature, 'astika' means 'one who believes in the authority of the Vedas' or 'one who believes in life after death'. ('Nastika' means the opposite of these).
Astika (IAST:astika) is a Sanskrit adjective (and noun) that is derived from asti ("it is or exists") meaning "believing" or "pious"; or "one who believes in the existence (of God, of another world, etc.)." Nastika (na (not) + astika) is its negative, literally meaning "not believing" or "not pious". As used in Hindu philosophy the differentiation between astika and nastika, refers to belief in Vedic authority, not belief or lack of belief in theism.
The Sanskrit term Astika ("pious, orthodox") refers to the systems of thought which admit the validity of the Vedas. Sanskrit asti means "there is", and Astika (per Pa?ini 4.2.60) derives from the verb, meaning "one who says 'asti'". Technically, in Hindu philosophy; the term Astika refers only to acceptance of authority of Vedas.
The three main schools of Indian philosophy that do not base their beliefs on the Vedas were regarded as heterodox or nastika:
* Buddhism
* Jainism
* Carvaka
At an early period, during the formation of the Upanisads and the rise of Buddhism and Jainism, we must envisage a common heritage of meditation and mental discipline practiced by renouncers with varying affiliations to non-orthodox (Veda-rejecting) and orthodox (Veda-accepting) traditions.... These schools [such as Buddhism and Jainism] are understandably regarded as heterodox (nastika) by orthodox (astika)
Hindu atheists treat Hinduism as a "Way of Life" but not as a religion. Their day-to-day lifestyle will be almost similar to most Hindu theists but they do not accept the existence of God. They live by their Hindu values and traditions. The words Hindus and Hinduism never existed until the Islamic invasion of India. Islamic invaders called the local population as Hindus which included a wide range of people with varied beliefs united by common ancestry, culture and Linguistic brotherhood.
Regards,
Kannan Kumaraswamy