Guru

The Advaya-Târaka-Upanishad (16) gives an esoteric explanation of the word guru, deriving it from syllable gu (indicating "darkness") and ru (indicating "dispeller"). Thus the guru is one who dispels the disciple's spiritual benightedness.

(...)

 

In the Kula-Arnava-Tantra, God Shiva addressing his divine spouse Devî, has this to say about realized masters as opposed to ordinary teachers:

There are many gurus, like lamps in house after house, bat hard to find, O Devî, is the guru who lights up all like the sun. (13.104)

There are many gurus who are proficient in the Vedas [revealed sacred knowledge] and the Shâstras [text-books], but hard to find, O Devî, is the guru who has attained to the supreme Truth. (13.105)

There are many gurus on Earth who give what is other than the Self, but hard to find in all the worlds, O Devî, is the guru who reveals the Self. (13.106)

Many are the gurus who rob the disciple of his wealth, but rare is the guru who removes the afflictions of the disciple. (13.108)

He is a [true] guru by whose very contact there flows the supreme Bliss (ânanda). The intelligent man should choose such a one as his guru and none other. (13.110)

.
from "The Yoga Tradition: Its History, Literature, Philosophy and Practice", by Georg Feuerstein