गुशब्दस्त्वन्धकारस्य रुशब्दस्तन्निवारकः ।
अन्धकारनिरोधित्वाद्गुरुरित्यभिधीयते ॥
gu-śabdas tv andhakārasya ru-śabdas tan-nivārakaḥ |
andhakāra-nirodhitvād gurur ity abhidhīyate ||
(Skanda Purāṇa)
“The syllable gu means darkness, and the syllable ru means its remover. A guru is so called because of his being a dispeller of darkness.”