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 133

ha-karena tu suryah syat-sa-karen-{e}[i]ndur-uchyate |"
surya-cha[a]ndramas[a]-{o}[u]raikyam hatha ity-abhidhiyate ||133||

हकारेण तु सूर्यः स्यात्सकारेण्{ए}[इ]न्दुरुच्यते ।"
सूर्याच[अ]न्द्रमस्[अ]{ओ}[उ]रैक्यं हठ इत्यभिधीयते ॥१३३॥

ha-kāreṇa tu sūryaḥ syāt-sa-kāreṇ-{e}[i]ndur-ucyate ।"
sūryā-ca[a]ndramas[a]-{o}[u]raikyaṁ haṭha ity-abhidhīyate ॥133॥

It is said that in the sound HA the sun is contained, in the sound SA the moon. |
Hatha-Yoga means uniting of sun and moon. ||133||


hakarena (हकारेण, hakāreṇa) = in the sound / character "ha" - (Ins.)
tu (तु, tu) = then (conclusion)
suryah (सूर्यः, sūryaḥ) = sun
syat (स्यात्, syāt) = 3rd person, optative of being (Verb)
sakarena (सकारेण, sakāreṇa) = in the sound / character " sa" - (Ins.)
induh (इन्दुः, induḥ) = moon
uchyate (उच्यते, ucyate) = is called, it is said, it is claimed
surya (सूर्या, sūryā) = sun
chandramas (चन्द्रमस्, candramas) = moon
aikyam (ऐक्यम्, aikyam) = unity, harmony, equality, uniting
hatha (हठ, haṭha) = Hatha Yoga
iti (इति, iti) = so, therefore
abhidhiyate (अभिधीयते, abhidhīyate) = has the meaning, is so explained, means