in Indian philosophy, any orthodox school of thought, defined as one that accepts the authority of the Vedas (sacred scriptures of ancient India); the superiority of the Brahmans (the class of priests), who are the expositors of the law (dharma); and a society made up of the four traditional classes (varna). The six orthodox philosophic systems are those of Sāṃkhya and Yoga, Nyāya and Vaiśeṣika, and Mīmāṃsā and Vedānta.

The term āstika comes from the Sanskrit asti, which means “there is.” Contrasted to the āstika systems are the nāstika (Sanskrit: from na asti, “there is not”), the individuals and schools that do not accept the authority of the Veda, the system of the four classes, and the superiority of the Brahmins. Included among the nāstika schools are the Buddhists, Jainas, the ascetic Ājīvikas, and the materialistic Cārvākas.