Sahaja (Sanskrit: सहज sahaja) means spontaneous enlightenment in Indian and Tibetan Buddhist spirituality. Sahaja practices first arose in Bengal during the 8th century among Buddhist yogis called Sahajiya siddhas.
Ananda Coomaraswamy describes its significance as "the last achievement of all thought", and "a recognition of the identity of spirit and matter, subject and object", continuing "There is then no sacred or profane, spiritual or sensual, but everything that lives is pure and void."
The Sanskrit [and the Tibetan, which precisely follows it] literally means: 'born or produced together or at the same time as. Congenital, innate, hereditary, original, natural (...by birth, by nature, naturally...)'.
Etymologically, saḥ- means 'together with', and ja derives from the root jan, meaning 'to be born, produced, to occur, to happen'. The Tibetan lhan cig tu skye ba is an exact etymological equivalent of the Sanskrit. Lhan cig means 'together with', and skye ba means 'to be born, to arise, to come about, to be produced'. The Tibetan can function as verbal phrase, noun, or adjective.
The Buddhist siddha Saraha (8th century CE) was the founder of the Buddhist movement termed "Sahajayana" which flourished in Odisha  and Bengal. Sahajiya mahasiddhas like Saraha, Kanha, Savari and Luipa were tantric Buddhists who expounded their beliefs in songs and dohas in the Apabhraṃśa languages  and Bengali. Many of the songs in this tradition are preserved in the Charyapada.
Sahajiyas such as Saraha believed that enlightenment could be achieved in this lifetime, by laypersons living in samsara. Though he was a famed Buddhist sage,. The sahajiyas practiced a form of ritual union which was supposed to bring the female and male elements together in balance.
Saraha and his disciples were also master practitioners of Mahamudra meditation, and Saraha composed a famous Mahamudra meditation text along with his 'Three Cycles of Doha', a series of yogic songs. Sahajiyas also criticized the Hindu caste system. Sahajayana Buddhism became very popular in the Pala Empire, especially among commoners.
One of the classic texts associated with the Sahajiya Buddhists is the Hevajra Tantra. The tantra describes four kinds of Joy (ecstasy):
The siddhi, Indrabhuti, wrote a commentary on Sahaja teachings called the Sahajasiddhipaddhati.
Sahaja is one of the four keywords of the Nath sampradaya along with Svecchachara, Sama, and Samarasa. Sahaja meditation and worship was prevalent in Tantric traditions common to Hinduism and Buddhism in Bengal as early as the 8th–9th centuries. The British Nath teacher Mahendranath wrote:
Man is born with an instinct for naturalness. He has never forgotten the days of his primordial perfection, except insomuch as the memory became buried under the artificial superstructure of civilization and its artificial concepts. Sahaja means natural... The tree grows according to Sahaja, natural and spontaneous in complete conformity with the Natural Law of the Universe. Nobody tells it what to do or how to grow. It has no swadharma or rules, duties and obligations incurred by birth. It has only svabhava - its own inborn self or essence - to guide it. Sahaja is that nature which, when established in oneself, brings the state of absolute freedom and peace.
The concept of a spontaneous spirituality entered Hinduism with Nath yogis such as Gorakshanath and was often alluded to indirectly and symbolically in the twilight language (sandhya bhasa) common to sahaja traditions as found in the Charyapada and works by Matsyendranath and Daripada. It influenced the bhakti movement through the Sant tradition, exemplified by the Bauls of Bengal, Dnyaneshwar, Meera, Kabir and Guru Nanak, the founder of Sikhism.
Yoga in particular had a quickening influence on the various Sahajiya traditions. The culture of the body (kāya-sādhana) through processes of Haṭha-yoga was of paramount importance in the Nāth sect and found in all sahaja schools. Whether conceived of as 'supreme bliss' (Mahā-sukha), as by the Buddhist Sahajiyās, or as 'supreme love' (as with the Vaiṣṇava Sahajiyās), strength of the body was deemed necessary to stand such a supreme realisation.
The Vaishnava-Sahajiya sect became popular in 17th century Bengal. It sought religious experience through the five senses. The divine relationship between Krishna and Radha (guises of the divine masculine and divine feminine) had been celebrated by Chandidas (Bangla: চন্ডীদাস) (born 1408 CE), Jayadeva (circa 1200 CE) and Vidyapati (c 1352 - c 1448) whose works foreshadowed the rasas or "flavours" of love. The two aspects of absolute reality were explained as the eternal enjoyer and the enjoyed, Kṛṣṇa and Rādhā conceived of as ontological principles of which all men and women are physical manifestations, as may be realised through a process of attribution (Aropa), in which the sexual intercourse of a human couple is transmuted into the divine love between Kṛṣṇa and Rādhā, leading to the highest spiritual realisation, the state of union or Yugala. The element of love, the innovation of the Vaiṣṇava Sahajiyā school, is essentially based on the element of yoga in the form of physical and psychological discipline.
Vaisnava-Sahajiya is a synthesis and complex of traditions that, due to its sexual tantric practices, was perceived with disdain by other religious communities and much of the time was forced to operate in secrecy. Its literature employed an encrypted and enigmatic style. Because of the necessity of privacy and secrecy, little is definitively known about their prevalence or practices.
The sahaja-siddhi or the siddhi or "natural accomplishment" or the "accomplishment of the unconditioned natural state" was also a textual work, the Sahaja-Siddhi revealed by Dombi Heruka (Skt. Ḍombi Heruka or Ḍombipa) one of the eighty-four Mahasiddhas. The following quotation identifies the relationship of the 'mental flux' (mindstream) to the sahaja-siddhi. Moreover, it must be remembered that though Sundararajan & Mukerji (2003: p. 502) use a masculine pronominal the term 'siddha' is not gender-specific and that there were females, many as senior sadhakas, amongst the siddha communities:
The practitioner is now a siddha, a realized soul. He becomes invulnerable, beyond all dangers, when all forms melt away into the Formless, "when surati merges in nirati, japa is lost in ajapā" (Sākhī, "Parcā ko Aṅga," d.23). The meeting of surati and nirati is one of the signs of sahaja-siddhi; surati is an act of will even when the practitioner struggles to disengage himself from worldly attachments. But when his worldliness is totally destroyed with the dissolution of the ego, there is nirati, cessation of the mental flux, which implies cessation of all willed efforts. Nirati (ni-rati) is also cessation of attractions, since the object of attraction and the seeker are now one. In terms of layayoga, nirati is dissolution of the mind in "Sound," nāda.
Sahaja samadhi is a state in which the silent awareness of the subject is operant along with (simultaneously with) the full use of the human faculties.
Kevala nirvikalpa samadhi is temporary, [web 1][web 2] whereas sahaja nirvikalpa samadhi is a continuous state throughout daily activity. This state seems inherently more complex than samadhi, since it involves several aspects of life, namely external activity, internal quietude, and the relation between them. It also seems to be a more advanced state, since it comes after the mastering of samadhi.[note 1][note 2]