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 136

layat-samprapyate saukhyam sva-atma-anadam param padam |"
yoni-madhye maha-ksetre japa-bandhuka-sannibham ||136||

लयात्संप्राप्यते सौख्यं स्वात्माअनदं परं पदम् ।"
योनिमध्ये महाक्षेत्रे जपाबन्धूकसंनिभम् ॥१३६॥

layāt-saṁprāpyate saukhyaṁ sva-atma-ānadaṁ paraṁ padam ।"
yoni-madhye mahā-kṣetre japā-bandhūka-saṁnibham ॥136॥

Through the sublime state of Laya bliss will be achieved out of itself. |
This is localised in the holy shrine in the middle of the Vagina (Yoni),
comparable with the Hibiskus- or Dupurmoni- blossom. ||136||


layat (लयात्, layāt) = through Laya (Abl.), Samadhi, goal of Yoga, dissolving
samprapyate (सम्प्राप्यते, samprāpyate) = fully reach, overcome (pres. pass)
saykhyam (सय्ख्यम्, saykhyam) = health, well-being, happiness, joy, fun, pleasure
sva (स्व, sva) = own
atma (आत्म, ātma) = self, soul, the true nature
ananda (आनन्द, ānanda) = bliss, joy
sva-atma-anandam (स्वात्माअनन्दं, sva-atma-ānandaṁ) = the bliss of it’s true natur, bliss out of itself,
param (परम्, param) = highest, sublime
padam (पदम्, padam) = stage, rank, position
yoni (योनि, yoni) = female sexual organs, typical symbol for divine energy of creation, Vagina
madhye (मध्ये, madhye) = in the middle, centre (Loc.)
maha (महा, mahā) = great, magnificent
ksetre (क्षेत्रे, kṣetre) = in the place, region (Loc.)
maha-ksetre (महाक्षेत्रे, mahā-kṣetre) = holy shrine
japa (जपा, japā) = China rose, Hibiskus
bandhuka (बन्धूक, bandhūka) = Dupurmoni flower, lat: Pentapetes Phoenicea
sannibha (संनिभ, saṁnibha) = similar, like