Samadhi

Samādhi, in the Indian religions, is a state of meditative consciousness. In many such traditions, the cultivation of samādhi through various meditation methods is essential for the attainment of spiritual liberation.
In Buddhism, it is the last of the eight elements of the Noble Eightfold Path. In the Ashtanga Yoga tradition, it is the eighth and final limb identified in the Yoga Sutras of Patanjali. In Jain meditation, samadhi is considered one of the last stages of the practice just prior to liberation.
In the oldest Buddhist sutras, on which several contemporary western Theravada teachers rely, it refers to the development of an investigative and luminous mind that is equanimous and mindful. In the yogic traditions and the Buddhist commentarial tradition, on which the Burmese Vipassana movement and the Thai Forest tradition rely, it is interpreted as a meditative absorption or trance attained by the practice of dhyāna.

Definitions

Samadhi may refer to a broad range of states. A common understanding regards samadhi as meditative absorption:

Sarbacker: samādhi is meditative absorption or contemplation.
Diener, Erhard & Fischer-Schreiber: samādhi is a non-dualistic state of consciousness in which the consciousness of the experiencing subject becomes one with the observing object.
Shivananda: "When the mind is completely absorbed in one object of meditation, it is termed Samadhi."

In a Buddhist context, samadhi is a state of intensified awareness and focus:

Dogen: "The Buddha says: 'When you monks unify your minds, the mind is in samadhi. Since the mind is in samadhi, you know the characteristics of the creation and destruction of the various phenomena in the world When you gain samadhi, the mind is not scattered, just as those who protect themselves from floods guard the levee.
Richard Shankman: "The term samadhi basically means 'undistractedness. It may be viewed as "an exclusive focus on a single object," but also as "a broader state of awareness in which the mind remains steady and unmoving, yet aware of a wide range of phenomena around the meditation object." According to Shankman, the related term cittas'ekaggata may be rendered as "one-pointedness," fixated on a single object, but also as "unification of mind," in which mind becomes very still but does not merge with the object of attention, and is thus able to observe and gain insight into the changing flow of experience.
Dan Lusthaus: "Samadhi provides the methodology and context within which experience is to be examined Samadhi, by training, focusing/collecting, cleansing and calming the mind facilitates things being finally known and seen just as they are.
Keren Arbel: "Samadhi is depicted as a broad field of awareness, knowing but non-discursive a stable, discerning and focused mind."
Tilmann Vetter argues that the second, third and fourth dhyana in Buddhism, samma-samadhi, "right samadhi," build on a "spontaneous awareness" and equanimity which is perfected in the fourth dhyana.

In Hinduism, samadhi is also interpreted as the identification with the Absolute:

Paramahansa Yogananda: A soundless state of breathlessness. A blissful super consciousness state in which a yogi perceives the identity of the individualized Soul and Cosmic Spirit.
Etymology

Sanskrit

Various interpretations for the term's etymology are possible, either with the root sam or sama. According to Dan Lusthaus, samadhi refers to either bringing to consciousness the samskaras, or meditative concentration on a meditation object:

sam, "to bring together"; adhi, "to place on, put, to impregnate, to give, to receive": the bringing together of cognitive conditions," "bringing the buried latencies or samskaras into full view," so "the obscure and hidden become clear objects of cognition," "the womb through which insight is born."
sama, "the same, equalized, the convergence of two distinct things based on some commonality"; adhi, "higher, better, most skilfully achieved": "the skillful unification of mind and object," "the mental equanimity conducive to and derived from attention perfectly focused on its object." "ometimes treated as synonymous with ekacitta, 'one-focused mind,' i.e. mind completely focused on and at one with its object."

Etymologies for sam-''ā-dhā include:

sam-ā-dhā’: "'to collect' or 'to bring together', thus suggesting the concentration or unification of the mind"; generally translated as "concentration."
sam-ā''-dhā: "to hold together, to concentrate upon."
sam, "completely"; ā, "the return towards the subject"; dha, "maintaining together: "to assemble completely"; "the tension borne between two poles of existence is reduced to zero."
sam, "together" or "integrated"; ā, "towards"; dhā, "to get, to hold": to acquire integration or wholeness, or truth ;
sam, "together"; ā, "toward"; stem of dadhati, "puts, places": a putting or joining;

Particular Hindu/yoga interpretations include:

sam, "perfect" or "complete"; dhi, "consciousness": "all distinctions between the person who is the subjective meditator, the act of meditation and the object of meditation merge into oneness" ;
sam, "with"; ādhi, "Lord": Union with the Lord ;
sama, "equanimous"; dhi, "buddhi or the intellect": equanimous intellect, non-discriminating intellect ;
sama, "balance"; ādi, "original": " a state that is equal to the original state, which is the state that prevailed before we came into existence"; "original balance" (Kamlesh D. Patel.
Chinese

Common Chinese terms for samādhi include the transliterations sanmei and sanmodi, as well as the translation of the term literally as ding. Kumarajiva's translations typically use sanmei, while the translations of Xuanzang tend to use ding. The Chinese Buddhist canon includes these, as well as other translations and transliterations of the term.

Buddhism

''Samma-samādhi'' and ''dhyāna'' (jhāna)

Samma-samadhi, "right samadhi," is the last of the eight elements of the Noble Eightfold Path. When samadhi is developed, things are understood as they really are.
Samma-samadhi is explicated as dhyana, which is traditionally interpreted as one-pointed concentration. Yet, in the stock formula of dhyāna ''samādhi is only mentioned in the second dhyana, to give way to a state of equanimity and mindfulness, in which one keeps access to the senses in a mindful way, avoiding primary responses to the sense-impressions.
The origins of the practice of dhyāna are a matter of dispute. According to Crangle, the development of meditative practices in ancient India was a complex interplay between Vedic and non-Vedic traditions. According to Bronkhorst, the four rūpa jhāna may be an original contribution of the Buddha to the religious landscape of India, which formed an alternative to the painful ascetic practices of the Jains, while the arūpa jhāna'' were incorporated from non-Buddhist ascetic traditions. Alexander Wynne argues that dhyāna was incorporated from Brahmanical practices, in the Nikayas ascribed to Alara Kalama and Uddaka Ramaputta. These practices were paired to mindfulness and insight, and given a new interpretation. Kalupahana also argues that the Buddha "reverted to the meditational practices" he had learned from Alara Kalama and Uddaka Ramaputta.

The ''rupa jhānas''

The ''arupas''

Appended to the jhana-scheme are four meditative states, referred to in the early texts as arupas or as āyatana. They are sometimes mentioned in sequence after the first four jhānas and thus came to be treated by later exegetes as jhānas. The immaterial are related to, or derived from, yogic meditation, and aim more specific at concentration, while the jhanas proper are related to the cultivation of the mind. The state of complete dwelling in emptiness is reached when the eighth jhāna is transcended. The four arupas are:

fifth jhāna: infinite space,
sixth jhāna: infinite consciousness,
seventh jhāna: infinite nothingness,
eighth jhāna: neither perception nor non-perception.

Although the "Dimension of Nothingness" and the "Dimension of Neither Perception nor Non-Perception" are included in the list of nine jhanas attributed to the Buddha, they are not included in the Noble Eightfold Path. Noble Path number eight is "Samma Samadhi", and only the first four Jhanas are considered "Right Concentration". When all the jhanas are mentioned, the emphasis is on the "Cessation of Feelings and Perceptions" rather than stopping short at the "Dimension of Neither Perception nor Non-Perception".

Theravāda

Samadhi as concentration

According to Gunaratana, the term 'samādhi' derives from the roots 'sam-ā-dhā', which means 'to collect' or 'bring together', and thus it is generally translated as "concentration." In the early Buddhist texts, samādhi is also associated with the term samatha. In the commentarial tradition, samādhi is defined as ekaggata, one-pointedness of mind.
Buddhagosa defines samādhi as "the centering of consciousness and consciousness concomitants evenly and rightly on a single object the state in virtue of which consciousness and its concomitants remain evenly and rightly on a single object, undistracted and unscattered". According to Buddhaghosa, the Theravada Pali texts mention four attainments of samādhi:

Momentary concentration : a mental stabilization which arises during samatha meditation.
Preliminary concentration : arises out of the meditator's initial attempts to focus on a meditation object.
Access concentration : arises when the five hindrances are dispelled, when jhāna is present, and with the appearance the 'counterpart sign'.
Absorption concentration : the total immersion of the mind on its meditation of object and stabilization of all four jhānas.

According to Buddhaghosa, in his influential standard-work Visuddhimagga, samādhi is the "proximate cause" to the obtainment of wisdom. The Visuddhimagga describes 40 different objects for meditation, which are mentioned throughout the Pali canon, but explicitly enumerated in the Visuddhimagga, such as mindfulness of breathing and loving kindness.