A guru is a spiritual teacher, someone who leads a disciple to wisdom and self-realization, imparts knowledge on the disciple, or guides the disciple to divinity. The word guru is commonly used in the Hindu, Buddhist, and Sikh traditions to indicate a religious teacher. The word originates in Sanskrit, and is formed by the syllables gu and ru. Gu indicates darkness, and ru indicates destruction. Thus, when translated directly, guru means ‘dispeller of darkness.’ This simple etymological explanation is supplemented by the postulation of numerous texts discussing the nature and role of a guru. A popular example of such a discussion would be that of the etymology of the syllables gu and ru to indicate the juxtaposition of light and dark, where ignorance is dark and knowledge, particularly spiritual knowledge, is light.

The notion of the guru dates back to a collection of ancient Indian texts known as the Upanishads. Other ancient Indian texts that discuss the concept of guru include the Bhagavad Gita, a section of the Indian epic, the Mahabharata. The Guru Gita and, less directly, the Ramayana are other ancient Indian texts addressing the role of the guru, and the disciple known as the shishya. In these texts, the ideal guru-shishya relationship is exemplified in the characters. In the Bhagavad Gita, Krishna plays the role of guru to the warrior prince Arjuna, guiding him through his uncertainty regarding morality and duty. In the Ramayana, the guru-shishya relationship is demonstrated through the hero Rama and the monkey deity Hanuman. In the Guru Gita, Lord Shiva is the guru, while his female consort Parvati is the shishya.

Within the Hindu tradition, reverence for a guru can range from a deep respect, to the assertion that the guru is an embodiment of a god on earth. In most cases, the guidance of a guru is considered essential in reaching moksha, or spiritual devotion and liberation, which is the final of four main goals in the schema of traditional Indian life. In the Hindu tradition a guru will likely give the disciple a mantra, a powerful sound or statement, as one of the many devices necessary in approaching enlightenment. The guru will guide the disciple in religious rituals and ceremonies with the goal of awakening the sleeping snake of shakti, or spiritual knowledge. In many cases the guru is revered above the god, as the guru is the one who leads the disciple to the encounter with the god.

In the Buddhist tradition the guru has a similar identity to that of the Hindu guru, although the teachings imparted are obviously different. Buddhist gurus are inspirational, highly respected teachers in the path to Enlightenment. In some branches of Buddhism, the guru is considered the embodiment of Buddha, a being known as a Boddhisattva. Tibetan Buddhism asserts that, without the guidance of a guru, there can be no true insight or experience — and certainly no enlightenment. The concept of the guru in Sikhism, while retaining the identity of the guru as a spiritual teacher, also abstracts the idea of guru to relate to knowledge imparted by any means or medium.

In the west, the concept of the guru is looser, and extends beyond teachers of spirituality and religion. Anyone who acquires followers and who is considered an authority in their arena may be called a guru. Many westerners, however, do retain the religious association with the concept of the guru. Well-known gurus in the west include Chogyam Trungpa Rinpoche and Paramahansa Yogananda.