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 138

anim-adi-padam prapya rajate raja-yogatah |"
prana-apana-samayogo jneyam yoga-chatushtayam ||138||

अणिमादिपदं प्राप्य राजते राजयोगतः ।"
प्राणापानसमायोगो ज्ञेयं योगचतुष्टयम् ॥१३८॥

aṇim-ādi-padaṁ prāpya rājate rāja-yogataḥ ।"
prāṇa-apāna-samāyogo jñeyaṁ yoga-catuṣṭayam ॥138॥

If Raja-Yoga is reached, supernatural powers (Siddhi) are achievable. |
Typical for the fourfold Yoga is always the unification of ascending energy (Prana) with descending energy(Apana). ||138||


animan (अणिमन्, aṇiman) = One of the supernatural powers (Siddhi), the power to shrink to the size of an Atom.
adi (आदि, ādi) = and so to
animan-adi (अणिमनादि, aṇiman-ādi) = the supernatural, powers, Siddhi
pada (पद, pada) = rank, state, stage, step
prapya (प्राप्य, prāpya) = reachable, achievable, to attain
rajate (राजते, rājate) = rule, reign, govern
raja-yogat (राजयोगात्, rāja-yogāt) = through Raja-Yoga
prana (प्राण, prāṇa) = ascending life force, principle of breathing
apana (अपान, apāna) = descending life force, principle of excretion
samayoga (समायोग, samāyoga) = unification, combination
jneyam (ज्ञेयम्, jñeyam) = is known
yoga (योग, yoga) = Yoga
chatushtayam (चतुष्टयम्, catuṣṭayam) = fourfold, consisting of 4.
yoga-chatushtayam (योगचतुष्टयम्, yoga-catuṣṭayam) = the fourfold Yoga; in sentence 1.129b-130a Mantra-, Laya-, Hatha- and Raja-Yoga are described as the four pieces of a Maha-Yoga.