The Yoga Shikha Upanishad probably originated from the 13th century. This extract from the first chapter perhaps present the earliest known tantric view of Hatha yoga and Raja yoga. This Upanishad is also the oldest text in which the two terms are defined.

Upanishads

Raja yoga described as the union of male semen (Retas (रेतस्, Retas)) with female vaginal secretion (Rajas (रजस्, Rajas)) (YSU I.137).This interpretation of Raja yoga was often literally translated particularly in tantric Kaula (कौल, Kaula) schools. In other tantric traditions, Raja-Yoga (रजयोग, Raja-Yoga) is often translated as the union of Shiva (शिव, Śiva) and Shakti (शक्ति, Śakti), the male and female beings.Modern interpretations of the term Raja yoga usually gloss over these tantric ideas.

Yoga-Shikh[a]-{O}[U]panishat

Sentence 137

rajo vasati jantunam devi-tattvam samavritam |"
rajaso retaso yoga-adraja-yoga iti snritah ||137||

रजो वसति जन्तूनां देवीतत्त्वं समावृतम् ।"
रजसो रेतसो योगाद्राजयोग इति स्मृतः ॥१३७॥

rajo vasati jantūnāṁ devī-tattvaṁ samāvr̥tam ।"
rajaso retaso yoga-adrāja-yoga iti smr̥taḥ ॥137॥

Hidden in all living beings the essence of the goddess lies dormant as vaginal secretion (Rajas). |
Hence it is handed down that Raja-Yoga evolves from the unification of male semen (Retas) and female vaginal secretion (Rajas). ||137||


rajas (रजस्, rajas) = menstrual blood, female principle, vaginal secretion
vasati (वसति, vasati) = stay, live
jantunam (जन्तूनाम्, jantūnām) = living being, creatures (Gen. plural)
devi (देवी, devī) = Devi, goddess
tattvam (तत्त्वम्, tattvam) = true principle, the essence
samavritam (समावृतम्, samāvr̥tam) = hidden, covered, concealed
rajaso (रजसो, rajaso) = menstrual blood, female principle, vaginal secretion
retas (रेतस्, retas) = flow of semen, sperm, sperm fluid
yogat (योगात्, yogāt) = of the unity (Abl.)
rajayoga (राजयोग, rājayoga) = Raja-Yoga, name of this path of Yoga
iti (इति, iti) = hence, this is why
snritah (स्मृतः, smr̥taḥ) = teached, explained, handed down