The issue of theism vis-à-vis atheism, in the ordinary senses of the English words, played an important role in Indian thought. The ancient Indian tradition, however, classified the classical systems (darshanas) into orthodox (astika) and unorthodox (nastika). Astika does not mean theistic, nor does nastika mean atheistic. Panini, a 5th-century-BCE grammarian, stated that the former is one who believes in a transcendent world (asti paralokah) and the latter is one who does not believe in it (nasti paralokah).

Jitendra N. Mohanty in: Indian philosophy, britannica.com