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 131

hansa-hans-eti mantro-.ayam sarvair-jivash-cha japyate |"
guruvakyatsushunnayam viparito bhavej-japah ||131||

हंसहंसेति मन्त्रो.अयं सर्वैर्जीवश्च जप्यते ।"
गुरुवाक्यात्सुषुम्नायां विपरीतो भवेज्जपः ॥१३१॥

haṁsa-haṁs-eti mantro-.ayaṁ sarvair-jīvaś-ca japyate ।"
guruvākyātsuṣumnāyāṁ viparīto bhavej-japaḥ ॥131॥

this way all living beings do chant the Mantra "hamsa hamsa". |
The other way around the Yogi should recite this Mantra – after instruction by his teacher - in the Sushumna. ||131||


hansa (हंस, haṁsa) = hamsa
iti (इति, iti) = this way, so
mantrah (मन्त्रः, mantraḥ) = mantra
ayam (अयम्, ayam) = this, this one
sarvais (सर्वैस्, sarvais) = through all (pl. Inst.)
jivas (जीवास्, jīvās) = living beings, souls
cha (च, ca) = and
japayate (जपयते, japayate) = chant repeatedly, recite repeatedly
guru (गुरु, guru) = Guru, Teacher
vakyat (वाक्यात्, vākyāt) = said, instructed, directed
sushunnayam (सुषुम्नायाम्, suṣumnāyām) = in the Sushumna (f. Loc.), main energy channel along the spine
viparitah (विपरितः, viparitaḥ) = reversed, the wrong way around
bhavet (भवेत्, bhavet) = he should, the Yogi should
japa (जप, japa) = chant repeatedly, recite repeatedly