A yogi is a practitioner of Yoga,  including a sannyasin or practitioner of meditation in Indian religions.  The feminine form, sometimes used in English, is yogini.
Yogi has since the 12th century CE also denoted members of the Nath siddha tradition of Hinduism,  and in Hinduism, Buddhism and Jainism, a practitioner of tantra.   In Hindu mythology, the god Shiva and the goddess Parvati are depicted as an emblematic yogi–yogini pair. 
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In Classical Sanskrit, the word yogi (Sanskrit: masc yogī, योगी; fem yoginī ) is derived from yogin, which refers to a practitioner of yoga. Yogi is technically male, and yoginī is the term used for female practitioners.  The two terms are still used with those meanings today, but the word yogi is also used generically to refer to both male and female practitioners of yoga and related meditative practices belonging to any religion or spiritual method.
The term yogini is also used for divine goddesses and enlightened mothers, all revered as aspects of the mother goddess, Devi. 
A yogi, states Banerjea, should not be confused with someone practicing asceticism and excessive self-mortification.  : 297
In Hinduism the term yogi refers to an adherent of yoga. 
The earliest evidence of yogis and their spiritual tradition, states Karel Werner,  is found in the Keśin hymn 10.136 of the Rigveda, [note 1] though with the terminology of Rudra who evolved into Shiva worshipped as the lord of Yoga in later Hinduism.  The Hindu scripture Rigveda uses words of admiration for the Yogis, whom it refers to as Kesin, and describes them as follows (abridged): 
Carrying within oneself fire and poison, heaven and earth, ranging from enthusiasm and creativity to depression and agony, from the heights of spiritual bliss to the heaviness of earth-bound labor. This is true of man in general and the [Vedic] Keśin in particular, but the latter has mastered and transformed these contrary forces and is a visible embodiment of accomplished spirituality. He is said to be light and enlightenment itself. The Keśin does not live a normal life of convention. His hair and beard grow longer, he spends long periods of time in absorption, musing and meditating and therefore he is called "sage" (muni). They wear clothes made of yellow rags fluttering in the wind, or perhaps more likely, they go naked, clad only in the yellow dust of the Indian soil. But their personalities are not bound to earth, for they follow the path of the mysterious wind when the gods enter them. He is someone lost in thoughts: he is miles away.— Karel Werner (1977), "Yoga and the Ṛg Veda: An Interpretation of the Keśin Hymn" 
The term yogin appears in Katyayana Shrauta-sutra and chapter 6 of Maitri Upanishad, where the implied context and meaning is "a follower of the Yoga system, a contemplative saint". 
The term sometimes refers to a person who belongs to the Natha tradition.  They usually belong to Shaiva tradition, but some Natha belong to the Vaishnava tradition.  In both cases, states David Lorenzen, they practice yoga and their principal god tends to be Nirguna, that is a god that is without form and semi-monistic,  influenced in the medieval era by the Advaita Vedanta school of Hinduism, Madhyamaka school of Buddhism, as well as Tantra and Yogic practices.  
The Yoga-Bhashya (400 CE),  the oldest extant commentary on the Yoga-Sutra offers the following fourfold classification of yogis:  
A yogi or yogini aspires to Brahmacharya (Sanskrit: ब्रह्मचर्य), which means celibacy if single, or non-cheating on one's partner.  
There have been two parallel views, in Hindu texts, on sexuality for a yogi and yogini. One view asserts restraint in sexual activity, towards monk- and nun-like asexuality, as transmutation away from worldly desires and onto a spiritual path.  It is not considered, states Stuart Sovatsky, as a form of moralistic repression but a personal choice that empowers the yoga practitioner to redirect his or her energies.  The second view, found particularly in Tantra traditions according to David Gordon White, asserts that sexuality is an additional means for a yogi or yogini to journey towards and experience the bliss of "one realized god-consciousness for oneself".  In the second view, sexuality is a yogic practice,  and one broadly revered through the lingam – yoni iconography of Shiva–Parvati, the divine yogi–yogini in Hindu mythology. 
Both a yogi and a philosopher are seekers of an absolute truth. But they differ in their modes of approach. A philosopher advances in the path of rational logic (theory) and wants to intellectually understand the Truth. A yogi advances in the path of self discipline (practice) and aspires to spiritually realize truth.
—Akshaya Banerjea, Philosophy of Gorakhnath 
A yogi or yogini lives by other voluntary ethical precepts called Yamas and Niyamas.   These include:   
According to David White,
[S]iddha means 'realized, perfected one', [note 2] a term generally applied to a practitioner ( sādhaka , sadhu ) who has, through his practice ( sadhana ), realized his dual goal of superhuman powers ( siddhis , 'realizations', 'perfections') and bodily immortality ( jivanmukti ). 
Archeological evidence suggests that in some contexts and regions, yogi of the Nath Siddha tradition were respected and recognized in India. For example, inscriptions suggest a general of the Yadava king Ramacandra donated a village to a yogi in 13th-century.  Near Mangalore, that later became a hub of Nath yogis, a monastery and temple was dedicated to yogis in the 10th-century. 
David Lorenzen states that the Nath yogis have been very popular with the rural population in South Asia, with medieval era "tales and stories about Nath yogis such as Gorakhnath, Matsyendra, Jalandhar, Gopichand, Bharthari, Kanhapa and Chaurangi" continuing to be remembered in contemporary times, in the Deccan, western and northern states of India and in Nepal. 
In some contexts, adds White, the term yogi has also been a pejorative term used in medieval India for a Nath siddha, particularly on the part of India's social, cultural and religious elites.  The term siddha has become a broad sectarian appellation, applying to Saiva-devotees in the Deccan ( Maheśvara siddhas), alchemists in Tamil Nadu (siddhars or sittars), a group of early Buddhist tantrikas from Bengal ( mahasiddhas , siddhacaryas), the alchemists of medieval India (rasa siddha), and a mainly north Indian group known as the Nath siddhas.  The Nath siddhas are the only still existing representatives of the medieval Tantric tradition, which had disappeared due to its excesses.  While the Nath siddhas enjoyed persistent popular success, they attracted the scorn of the elite classes. 
According to White, the term yogi, has "for at least eight hundred years, been an all-purpose term employed to designate those Saiva specialists whom orthodox Hindus have considered suspect, heterodox, and even heretical in their doctrine and practice".  The yoga as practiced by these Yogis, states White, is more closely identified in the eyes of those critics with black magic, sorcery and sexual perversions than with yoga in the conventional sense of the word. 
The Nath Yogis were targets of Islamic persecution in the Mughal Empire. The texts of Yogi traditions from this period, state Shail Mayaram, refer to oppressions by Mughal officials such as governor. The Mughal documents confirm the existence of Nath Yogis in each pargana (household neighborhoods), and their persecution wherein Nath Yogis were beheaded by Aurangzeb. 
According to David Lorenzen, the religious groups in Hinduism that militarized and took up arms following the Muslim conquest of India, to resist persecution, appeared among the Nath or Kanphata yogis, often called simply yogis or jogis. 
The warrior ascetics were institutionalized as a religious order by Gorakhnath and were expanding in the 13th century, after the establishment of the first Islamic Sultanate in India. They interacted and cooperated with fakirs of Sufi Muslims.  The yogis feature prominently in Delhi Sultanate and Mughal Empire period official documents, states David White, both in terms of impressing the ruling elite in the Muslim administration and awards of receiving land grants in some cases such as by Akbar, as well as those yogis who targeted the elite merchants and disrupted the business of administrative Islamic elites in urban areas.   In other cases, yogis from the Shaivism, Vaishnavism and Shaktism traditions of Hinduism marshaled armed resistance against the Mughal and British colonial armies.  
The history of Nath yogis has been diverse, such as in the 11th and 12th centuries, when Buddhists in South India converted to Nath siddha traditions and helped establish Shiva Hindu temples and monasteries. 
Chakras are various focal points used in a variety of ancient meditation practices, collectively denominated as Tantra, or the esoteric or inner traditions of Hinduism.
In Hinduism, Kundalini is a form of divine feminine energy believed to be located at the base of the spine, in the muladhara. It is an important concept in Śhaiva Tantra, where it is believed to be a force or power associated with the divine feminine or the formless aspect of the Goddess. This energy in the body, when cultivated and awakened through tantric practice, is believed to lead to spiritual liberation. Kuṇḍalinī is associated with Parvati or Adi Parashakti, the supreme being in Shaktism; and with the goddesses Bhairavi and Kubjika. The term, along with practices associated with it, was adopted into Hatha yoga in the 9th century. It has since then been adopted into other forms of Hinduism as well as modern spirituality and New age thought.
Tantra are the esoteric traditions of Hinduism and Buddhism that developed in India from the middle of the 1st millennium CE onwards. The term tantra, in the Indian traditions, also means any systematic broadly applicable "text, theory, system, method, instrument, technique or practice". A key feature of these traditions is the use of mantras, and thus they are commonly referred to as Mantramārga in Hinduism or Mantrayāna and Guhyamantra in Buddhism.
Yoga is a group of physical, mental, and spiritual practices or disciplines which originated in ancient India and aim to control (yoke) and still the mind, recognizing a detached witness-consciousness untouched by the mind (Citta) and mundane suffering (Duḥkha). There is a wide variety of schools of yoga, practices, and goals in Hinduism, Buddhism, and Jainism, and traditional and modern yoga is practiced worldwide.
Haṭha yoga is a branch of yoga. The Sanskrit word हठ haṭha literally means "force" and thus alludes to a system of physical techniques.
Tantric sex or sexual yoga refers to a wide range of practices carried on in Hindu and Buddhist tantra to exercise sexuality in a ritualized or yogic context, often associated with antinomian or impure elements, like consumption of alcohol, and offerings of impure substances like meat to fierce deities. In particular, sexual fluids have been viewed as "power substances" and used ritualistically, either externally or internally.
Sadhu, also spelled saadhu, is a religious ascetic, mendicant or any holy person in Hinduism and Jainism who has renounced the worldly life. They are sometimes alternatively referred to as yogi, sannyasi or vairagi.
Sādhanā is an ego-transcending spiritual practice. It includes a variety of disciplines in Hindu, Buddhist, Jain and Sikh traditions that are followed in order to achieve various spiritual or ritual objectives.
Nath, also called Natha, are a Shaiva sub-tradition within Hinduism in India and Nepal. A medieval movement, it combined ideas from Buddhism, Shaivism and Yoga traditions in India. The Naths have been a confederation of devotees who consider Shiva, as their first lord or guru, with varying lists of additional gurus. Of these, the 9th or 10th century Matsyendranath and the ideas and organization mainly developed by Gorakhnath are particularly important. Gorakhnath is considered the originator of the Nath Panth.
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.
A matha, also written as math, mutt, or mut, is a Sanskrit word that means 'institute or college', and it also refers to a monastery in Hinduism. An alternative term for such a monastery is adheenam.
Yogini is a female master practitioner of tantra and yoga, as well as a formal term of respect for female Hindu or Buddhist spiritual teachers in Indian subcontinent, Southeast Asia and Greater Tibet. The term is the feminine Sanskrit word of the masculine yogi, while the term "yogin" IPA: [ˈjoːɡɪn] is used in neutral, masculine or feminine sense.
Gorakhnath was a Hindu yogi, saint who was the influential founder of the Nath Hindu monastic movement in India and Nepal He is considered one of the two notable disciples of Matsyendranath. His followers are found in India, at the place known as Garbhagiri, which is in Ahmednagar in the state of Maharashtra. These followers are called yogis, Gorakhnathi, Darshani or Kanphata.
Dhyāna in Hinduism means contemplation and meditation. Dhyāna is taken up in Yoga practices, and is a means to samadhi and self-knowledge.
A sādhaka or sādhak or sādhaj, in Indian religions and traditions, such as Jainism, Buddhism, Hinduism and Yoga, is someone who follows a particular sādhanā, or a way of life designed to realize the goal of one's ultimate ideal, whether it is merging with one's eternal source, brahman, or realization of one's personal deity. The word is related to the Sanskrit sādhu, which is derived from the verb root sādh-, 'to accomplish'. As long as one has yet to reach the goal, they are a sādhaka or sādhak, while one who has reached the goal is called a siddha. In modern usage, sadhaka is often applied as a generic term for any religious practitioner. In medieval India, it was more narrowly used as a technical term for one who had gone through a specific initiation.
The Yoga Yajnavalkya is a classical Hindu yoga text in the Sanskrit language. The text is written in the form of a male-female dialogue between the sage Yajnavalkya and Gargi. The text consists of 12 chapters and contains 504 verses.
Yogi Nath is a Shaivism-related group of monks which emerged around the 13th-century. They are sometimes called Jogi or simply Yogi, and are known for a variety of siddha yoga practices. Their tradition is known as Nath Sampradaya.
The Yoga-kundalini Upanishad, also called Yogakundali Upanishad, is a minor Upanishad of Hinduism. The Sanskrit text is one of the 20 Yoga Upanishads, and is one of 32 Upanishads attached to the Krishna Yajurveda. In the Muktika canon, narrated by Rama to Hanuman, it is listed at number 86 in the anthology of 108 Upanishads.
The Yogashikha Upanishad is a Sanskrit text and one of the minor Upanishads of Hinduism. It is one of twenty Yoga Upanishads in the four Vedas.
David Gordon White is an American Indologist.
The Yogis of Vedic times left little evidence of their existence, practices and achievements. And such evidence as has survived in the Vedas is scanty and indirect. Nevertheless, the existence of accomplished Yogis in Vedic times cannot be doubted.
अथ यम-नियमाः अहिंसा सत्यमस्तेयं बरह्यछर्यम कश्हमा धृतिः दयार्जवं मिताहारः शौछम छैव यमा दश १७