Traditional Hindu philosophers emphasize the crucial importance of clearly understanding what is Hinduism proper and what are non-Hindu religious paths. You cannot claim to be a Hindu, after all, if you do not understand what it is that you claim to believe, and what it is that others believe.

"Vaidika" and "Avaidika"
One set of antonymous Sanskrit terms repeatedly employed by many traditional Hindu philosophers was vaidika and avaidika.

The word vaidika (or "Vedic" in English) means one who accepts the teachings of the Veda. It refers specifically to the unique epistemological stance taken by the traditional schools of Hindu philosophy, known as shabda-pramana, or employing the divine sound current of Veda as a means of acquiring valid knowledge. In this sense the word "vaidika" is employed to differentiate those schools of Indian philosophy that accept the epistemological validity of the Veda as apaurusheya, or a perfect authoritative spiritual source, eternal and untouched by the speculations of humanity, juxtaposed with the avaidika schools that do not ascribe such validity to the Veda.

In pre-Christian times, avaidika schools were clearly identified by Hindu authors as being specifically Buddhism, Jainism and the atheistic Charvaka school, all of whom did not accept the Veda. These three schools were unanimously considered non-Vedic, and thus non-Hindu (they certainly are geographically Indian religions, but they are not theologically/philosophically Hindu religions).

Views stated in the "Manava-dharma-shastra"
Manu, one of the great ancient law-givers of the Hindu tradition, states the following in his Manava-dharma-shastra:

"All those traditions and all those disreputable systems of philosophy that are not based on the Veda produce no positive result after death; for they are declared to be founded on darkness. All those doctrines differing from the Veda that spring up and soon perish are ineffectual and misleading, because they are of modern date." (XII, 95)

Stated in simpler terms, "vaidika" specifically refers to those persons who accept the Veda as their sacred scripture, and thus as their source of valid knowledge about spiritual matters.

Views stated in the "Sarva-darshana-samgraha"
In his famous compendium of all the known Indian schools of philosophy, the Sarva-darshana-samgraha, Madhava Acharya (a 14th century Advaita philosopher) unambiguously states that Charvakins (atheist empiricists), "Bauddhas" (Buddhists) and "Arhatas" (Jains) are among the non-Vedic, and thus non-Hindu, schools. Conversely, he lists Paniniya, Vaishnava, Shaiva and others among the Vedic, or Hindu, traditions. Likewise, in his Prasthanabheda, the well-known Madhusudana Sarasvati (fl. 17th century C.E.) contrasts all the mleccha (or "barbaric") viewpoints with Hindu views and says that the former are not even worthy of consideration, whereas the Buddhist views must at least be considered and debated.

The differentiation between "orthodox" and "heterodox", from a classical Hindu perspective, rests upon acceptance of the Vedic revelation, with the latter rejecting the sanctity of the Veda.

"Astika" and "Nastika"
As a further attempt to clearly distinguish between Hindu and non-Hindu, Hindu philosophers regularly used the Sanskrit terms astika and nastika. The two terms are synonymous with vaidika and avaidika, respectively. Astika refers to those who believe in the Vedas, nastika to those who reject the Vedas.

Under the astika category Hinduism would include any Hindu path that accepts the Veda, such as Vaishnavism, Shaivism, Shaktism, Advaita, Yoga, Nyaya, Mimamsa, among others. The nastika religions would include any religious tradition that does not accept the Veda: Buddhism, Jainism, Sikhism, Christianity, Islam, Baha'i, etc.

Thus when it came to the importance of unambiguously differentiating between the teachings of Hinduism and the teachings of non-Hindu religions, the most historically important sages of Hindu philosophical and theological thought are clear advocates of "Vaidika Dharma" - Hinduism - as a systematic, unitive tradition of spiritual expression.