Āstika (Sanskrit: आस्तिक āstika; "it (heaven) exists") and Nāstika (नास्तिक, nāstika; "it doesn't exist") are technical terms in Hinduism used to classify philosophical schools and persons, according to whether they accept the authority of the Vedas as supreme revealed scriptures, or not, respectively. By this definition, Nyāyá, Vaiśeṣika, Sāṃkhya, Yoga, Mimāṃsā and Vedānta are classified as āstika schools; and some schools like Cārvāka, Jainism and Buddhism are considered nāstika. The distinction is similar to the orthodox/heterodox distinction in the West.

In non-technical usage, the term āstika is sometimes loosely translated as "theist", while nāstika is translated as "atheist". However, this interpretation is distinct from the use of the term in Hindu philosophy. Notably even among the āstika schools, Sāṃkhya

The different usages of these terms are explained by Chatterjee and Datta as follows:

Āstika (IAST:āstika) 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 ."Nāstika (na (not) + āstika) is its negative, literally meaning "not believing" or "not pious". As used in Hindu philosophy the differentiation between āstika and nāstika refers to belief in Vedic authority, not belief or lack of belief in theism. As N. N. Bhattacharyya writes:

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