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 134

hathena grasyate jadyam sarva-dosha-samudbhavam |"
ksetrajnah paramatma cha tayoraikyam yada bhavet ||134||

हठेन ग्रस्यते जाड्यं सर्वदोषसमुद्भवम् ।"
क्षेत्रज्ञः परमात्मा च तयोरैक्यं यदा भवेत् ॥१३४॥

haṭhena grasyate jāḍyaṁ sarva-doṣa-samudbhavam ।"
kṣetrajñaḥ paramātmā ca tayoraikyaṁ yadā bhavet ॥134॥

When the individual and the universal sould became united, Hatha-Yoga |
then swallows the lethargy, the result of the basic characteristics of human beings. ||134||


hathena (हठेन, haṭhena) = through Hatha-Yoga - (Ins.)
grasyate (ग्रस्यते, grasyate) = eats up, swallows, lets vanish
jadyam (जाड्यम्, jāḍyam) = stupidity, lethary, weariness
sarva (सर्व, sarva) = all, each
dosha (दोष, doṣa) = Doshas (Vatta, Pitta, Kapha), basic characteristics of human beings
samudbhava (समुद्भव, samudbhava) = existence, origin, result
ksetrajnah (क्षेत्रज्ञः, kṣetrajñaḥ) = body awareness; physical self-perception, animated body
paramatma (परमात्मा, paramātmā) = universal soul
cha (च, ca) = and
tayoh (तयोः, tayoḥ) = to (Präposition: toward unity)
aikyam (ऐक्यम्, aikyam) = unity, harmony, equality
yada (यदा, yadā) = at this time, when
bhavet (भवेत्, bhavet) = bhuu (be, become) 3rd pers. optative