भारतीय दर्शन में आस्तिक शब्द तीन अर्थों में प्रयुक्त हुआ है-

• (1) जो लोग वेद को परम प्रमाण मानते हैं। इस परिभाषा के अनुयायी को आस्तिक कहलाते हैं और 6 दर्शन को आस्तिक दर्शन कहे जाते हैं।

• (2) जो आत्मा के अस्तित्व को स्वीकार करते हैं।

• (3) जो ईश्वर के अस्तित्व को स्वीकार करते हैं, तथा परलोक और मृत्युपश्चात् जीवन में विश्वास नहीं करते। इस परिभाषा के अनुसार केवल चार्वाक दर्शन जिसे लोकायत दर्शन भी कहते हैं, भारत में नास्तिक दर्शन कहलाता है और उसके अनुयायी नास्तिक कहलाते हैं।

Āstika and Nāstika are concepts that have been used to classify the schools of Indian philosophy by modern scholars, as well as some Hindu, Buddhist and Jain texts. The various definitions for āstika and nāstika philosophies have been disputed since ancient times, and there is no consensus. One standard distinction, as within ancient- and medieval-era Sanskrit philosophical literature, is that āstika schools accept the Vedas, the ancient texts of India, as fundamentally authoritative, while the nāstika schools do not. However, a separate way of distinguishing the two terms has evolved in current Indian languages like Telugu, Hindi and Bengali, wherein āstika and its derivatives usually mean 'theist', and nāstika and its derivatives denote 'atheism'. Still, philosophical tradition maintains the earlier distinction, for example, in identifying the school of Sāṃkhya, which is non-theistic, as āstika (Veda-affirming) philosophy, though "God" is often used as an epithet for consciousness (purusha) within its doctrine. Similarly, though Buddhism is considered to be nāstika, Gautama Buddha is considered an avatar of the god Vishnu in some Hindu denominations. Due to its acceptance of the Vedas, āstika philosophy, in the original sense, is often equivalent to Hindu philosophy: philosophy that developed alongside the Hindu religion.