Definition - What does Nastika mean?
Nastika is a Sanskrit term, traditionally used to refer to philosophical schools and people who do not have faith in the authority of the Vedas. Those who are nastika do not believe the Vedas to be supreme revealed scripture.
Nastika is the antonym of the Sanskrit word astika, which refers to the classical schools of philosophy which believe in the authority of the Vedas and sometimes also in the existence of God.
The terms nastika and astika both come from the root word asti, which means “it is” or “exists.”
Yogapedia explains Nastika
One interpretation of nastika and astika is that astika means “one who believes in the existence”, where "existence" refers to a soul distinct from the physical world, or Brahman, while nastika translates as "an unbeliever". Sometimes, nastika is loosely translated as “atheist” while astika is “theist”, however this is not very accurate, as there are astika schools, such as samkhya, that believe in the authority of the Vedas, but not in the existence of God.
The three main heterodox Indian schools of philosophy are considered nastika because their teachings are not based upon the Vedas. They are:
Within the Tantric traditions, there are schools which are both nastika and astika.