Yajur Veda · Shukla Yajur Veda (Sannyasa / Yoga Upanishad group) · 19 mantras
Advaya Taraka Upanishad
Advaya-tārakopaniṣad
Central theme: Taraka-brahma-yoga — the saviour Brahman + the famous gu-ru etymology
Just 19 mantras of dense yoga-Vedanta. "Taraka" — "that which ferries across" — names the inner light meditated upon between the eyebrows or at the heart-cave; the meditator becomes one with the saviour Brahman that ferries the jiva over samsara. Distinguishes purva-taraka (the prior, sa-vishaya — with object: trataka on inner stars, lights, suns) from uttara-taraka (the higher, nir-vishaya — objectless, manonmani, the unmesha of pure awareness). Lists the five-fold tri-lakshya-yoga (gazing at the tip of nose, midway, etc) and seals with the iconic guru-etymology: "gu means darkness, ru means its remover; the guru is so called because he removes the darkness."
Advaya Taraka 16
गुशब्दस्त्वन्धकारः स्यात् रुशब्दस्तन्निरोधकः । अन्धकारनिरोधित्वात् गुरुरित्यभिधीयते
Gu-śabdas tv andhakāraḥ syāt ru-śabdas tan-nirodhakaḥ, andhakāra-nirodhitvāt gurur ity abhidhīyate
The syllable "gu" denotes darkness; the syllable "ru" denotes its remover. Because he removes the darkness, he is called guru.
The guru is not a person but the function of darkness-removal — wherever that happens, there the guru is.