1. a Hindu or Sikh religious teacher or leader, giving personal spiritual guidance to his disciples
2. (often derogatory) a leader or chief theoretician of a movement, esp a spiritual or religious cult
3. (often facetious) a leading authority in a particular field ⇒ "a cricketing guru"
http://www.collinsdictionary.com/dictionary/english/guru
This word originated in India
If you want just an ordinary leader or guide, the ordinary words leader and guide (from Old English and French respectively) will do. But an extraordinary mentor requires a passage to India.
As long ago as the seventeenth century, English travelers to India brought back reports of gurus. In 1613 Samuel Purchas, the most famous travel writer of his time, first mentioned them in English: "They have others which they call Gurupi, learned Priests." He also tells of "A famous Prophet of the Ethnikes, named Goru."
In the Hindu and Buddhist religions, the guru is not merely a priest, nor merely a teacher, but a combination of the two. The relationship of guru to disciple is more intense and all-encompassing than that of teacher to student, more educational than that of priest to parishoner. A guru provides not just instruction but "guidance, protection, and grace." Until guru came into our language, we had no word for that most intense of leaders and teachers.
And it was not until 1940 that guru was exported from India to use in an English context. H. G. Wells wrote then, "I ask you, Stella, as your teacher, as your Guru, so to speak, not to say a word more about it," and in 1949 Arthur Koestler wrote, "My self-confidence as a Guru had gone."
Nowadays gurus and would-be gurus are plentiful in the English-speaking world. There are, for example, hardware gurus, magic gurus, game gurus, fantasy football gurus, and of course Internet gurus. For Yellowstone National Park, you can find a geyser guru.
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