गुशब्दस्त्वन्धकारः स्यात् रुशब्दस्तन्निरोधकः।
अन्धकारनिरोधित्वात् गुरुरित्यभिधीयते॥ १६॥The syllable gu means darkness, the syllable ru, he who dispels them,
Because of the power to dispel darkness, the guru is thus named.
আদায়াতরক উপনিষদ, শ্লোক-১৬
