gu-śabdas tv andhakārasya ru-śabdas tan-nivārakaḥ
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.”