A guru ( , ) is a person who is regarded as having great knowledge, wisdom and authority in a certain area, and uses it to guide others. The word comes from Sanskrit Gu, darkness, and Ru, light (prakash); literally a preceptor who shows others knowledge (light) and destroys ignorance (darkness).

It is also used for teacher or guide in the religious or sense, and is commonly used in Hinduism, Buddhism and Sikhism, as well as in some new religious movements. The guru is seen in these religions as a sacred conduit for wisdom and guidance, and finding a true guru is often held to be a prerequisite for attaining self-realization.

"Guru" also refers in Sanskrit to Brihaspati, a Hindu divine figure. In Vedic astrology, Guru or Brihaspati is believed to exert teaching influences. Indeed, in many Indian languages such as Hindi, the occidental Thursday is called either Brihaspativaar or Guruvaar (vaar meaning day of the week).

In contemporary India and Indonesia, the word "guru" is widely used with the general meaning of "teacher", including by schoolchildren. In Western usage, the meaning of guru has been extended to cover anyone who acquires followers, though not necessarily in an established school of philosophy or religion. Forsthoefel, T. and C. Humes. Gurus in America (2005) p.3. SUNY Publishers ISBN 0-7914-6574-8 In a further Western metaphorical extension, guru is used to refer to a person who has authority because of his or her perceived secular knowledge or skills.

Etymology

Guru is composed of the syllables 'gu' and 'ru', the former signifying 'darkness', and the latter signifying 'the destroyer of that 1', hence a guru is one characterized as someone who dispels spiritual ignorance (darkness), with spiritual illumination (light) -as per Advaya-Tãraka Upanishad (verse 16),

The word IAST: guru, a noun, means "teacher" or Spiritual Master in Sanskrit and in other languages derived from Sanskrit, such as Hindi, Marathi, Bengali, Gujarati and Nepali, or influenced by Sanskrit, such as Indonesian.

As a noun the word means the imparter of knowledge (jnana) . As an adjective, it means "heavy," or "weighty," in the sense of "heavy with knowledge,"Tirha, B. B. A Taste of Trascendence, (2002) p.161, Mandala Press. ISBN 1-886069-71-9

"Guru: a spiritual master; one who is heavy with knowledge of the Absolute and who removes nescience with the light of the divine."