Advayataraka Upanishad 14--18, verse 5)

Another popular etymology claims that the syllables gu (गु) and ru (रू), stand for darkness and light, respectively, providing the esoteric meaning that the guru is somebody who leads the disciple from the darkness of ignorance to the light of knowledge [1] [2].

In the sense mentioned here above, guru is used more or less interchangeably with "satguru" (literally: true teacher) and satpurusha. Compare also Swami. The disciple of a guru is called sishya or chela. Often a guru lives in an ashram or in a gurukula (the guru's household) together with his disciples. The lineage of a guru, spread by worthy disciples who carry on that guru's particular message, is known as the guru parampara or disciplic succession.

Gurus of several Hindu denominations, including the Surat Shabda Yoga are often referred to as Satgurus.

In the traditional sense, the word guru describes a relationship rather than an absolute and is used as a form of address only by a disciple addressing his master. Some Hindu denominations like BAPS Swaminarayan Sanstha hold that a personal relationship with a living guru, revered as the embodiment of God, is essential in seeking moksha. The guru is the one who guides his or her disciple to become a jivanmukta, liberated soul that achieves salvation in his or her lifetime through God-realization.

The role of the guru continues in the original sense of the word in such Hindu traditions as Vedanta, Yoga, Tantra and Bhakti schools. Indeed, it is now a standard part of Hinduism (as defined by the six Vedic streams and the Tantric Agamic streams) that a guru is one's spiritual guide on earth. In some more mystical traditions, it is believed that the guru could awaken dormant spiritual knowledge within the pupil, known as shaktipat.

In Hinduism the guru is considered a respected person with saintly qualities who enlightens the mind of his disciple, an educator from whom one receives the initiatory mantra, and one who instructs in rituals and religious ceremonies. The Vishnu Smriti and Manu Smriti regards the teacher, along with the mother and the father as the most venerable gurus (teachers) of an individual.

Some influential gurus in the Hindu tradition (there have been many) include Adi Shankaracharya, Shri Chaitanya Mahaprabhu, and Shri Ramakrishna. Other gurus whose legacy of continuing the Hindu yogic tradition grew in the 20th century were men like Shri Aurobindo Ghosh, Shri Ramana Maharshi, Swami Sivananda and Swami Chinmayananda.

In Indian culture not having a guru or a teacher (acharya) was looked down upon as being an orphan, and a sign of misfortune. The word anatha in Sanskrit means "the one without a teacher". An acharya is the giver of gyan (knowledge) in the form of shiksha (instruction). Guru also gives diksha (initiation) that is the spiritual awakening of the disciple by the grace of the Guru. Diksha is also considered to be the procedure of bestowing the divine powers of a Guru to the disciple, through which he progresses continuously on the path of divinity.

The institution of the guru has evolved various basic tenets of Indian culture and transmitted spiritual and fundamental knowledge. Gurus formed the axis of ancient educational system and ancient society, and enriched various fields of learning and culture by their creative thinking. In this lies the lasting significance of gurus and their contribution to the upliftment of mankind.

The origin of guru can be traced back as far as the early Upanishads, where the conception of the Divine Teacher on earth first manifested from its early Brahmin associations. Indeed, there is an understanding in some sects that if the devotee were presented with the guru and God, first he would pay respects to the guru since the guru had been instrumental in leading him to God. Saints and poets have sung the glory of the guru and the God such as Kabir and Brahmanand:

Guru Purnima is the day when the disciple wakes up in his fullness and expresses gratitude. The purpose of the Guru Purnima (or Poornima) celebration is to review the year and see in how much one has progressed in life and to renew one's determination and focus on the progress on the spriritual path.

Guru Puja (literally "worship of the guru") is a practice of making offerings to and requesting inspiration from the guru. Vows and commitments made by the disciple or chela, which might have lost their strength are renewed.

Guru Bhakti (literally "devotion to the guru") is considered important in many schools.

In the Upanishads five signs of sat guru (true guru) are mentioned.

In the presence of the satguru; Knowledge flourishes (Gyana raksha); Sorrow diminishes (Dukha kshaya); Joy wells up without any reason (Sukha aavirbhava); Abundance dawns (Samriddhi); All talents manifest (Sarva samvardhan).

According to Dr. Georg Feuerstein, the preceptors were traditionally treated with great reverence in correspondence with the perceived identification of the enlightened master with the transcendental Reality and that traditionally, gurus were granted excessive authority and strongly tended to be deified. He writes that, probably to counterbalance this, some Hindu schools began to emphasize that the real teacher is the transcendental Self.

The Shiva Samhita, a late medieval text on Hatha yoga, emphasizes the importance of the guru for liberation and asserts that the disciple is supposed to give his all his property and livestock to the guru upon diksha (initiation).

The Vishnu Smriti and Manu Smriti regards the Acharya (teacher/guru), along with the mother and the father as the most venerable individuals: The mother and father are the first "guru". The spiritual guru is the second.

The importance of finding a true guru is one of the tenants of Hinduism. Lord Krishna says to Arjuna:

The Advaya Taraka Upanishad states that the true teacher is well versed in the Veda, a devotee of Vishnu, free from envy, a knower of yoga and intent on yoga, and always having the nature of yoga. And the text continues by stating that he who is equipped with devotion to the teacher, who is a knower of the Self and possessing above mentioned characteristics is designated as a guru.

The Mundak Upanishad says to realize the supreme godhead one should surrender himself before the guru who knows the secrets of the Vedas.

The Maitrayaniya Upanishad warns against false teachers who deceive the naive.

The Kula-Arnava-Tantra states that there are many gurus who rob the disciple's wealth and few that remove the disciple's afflictions.

Gurus of the Surat Shabda Yoga traditions often are referred to as Satgurus or Sat (True) Gurus.

On the role of the guru, Swami Sivananda asks: "Do you realize now the sacred significance and the supreme importance of the Guru's role in the evolution of man? It was not without reason that the India of the past carefully tended and kept alive the lamp of Guru-Tattva. It is therefore not without reason that India, year after year, age after age, commemorates anew this ancient concept of the Guru, adores it and pays homage to it again and again, and thereby re-affirms its belief and allegiance to it. For, the true Indian knows that the Guru is the only guarantee for the individual to transcend the bondage of sorrow and death, and experience the Consciousness of the Reality."

Some Hindu gurus have warned against false teachers, have recommended the spiritual seeker to test the guru before accepting him, and have outlined criteria how to distinguish false from genuine ones:

According to Kranenborg (2002), the fact that some people follow false gurus is seen in India as due to their bad karma.

In his Encyclopedic Dictionary of Yoga (1990), Feuerstein writes that gurus occasionally exploit their followers because only few gurus enjoy full enlightenment. He further writes that many gurus in traditional Hinduism expect unquestioned obedience and constant service and possibly request hefty remuneration for initiation.

Continuing the work of the skeptic Sri Lankan professor Abraham Kovoor, the Indian amateur magician Basava Premanand (who is a former follower of several gurus) toured around in the villages of India to educate people by debunking gurus, godmen, and fakirs whom he considers frauds or self deceived, especially if they claim to perform paranormal feats, or to possess siddhis, or actively promote belief in miracles. The Ghandian rationalist, physicist, and educationalist H. Narasimhaiah founded The Committee to Investigate Miracles and Other Verifiable Superstitions in the 1970s to investigate the claims of miracles performed by gurus and godmen.

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