What exactly is Jhana?

What exactly is the theory of jhana

In simple terms, Jhana is to look at something clearly or closely. [Abhidharmartha Pradipika, vol 1, p 71] It can be something pertaining to the ultimate realities or a conceptual object.

Also see my answer: What to make of Jhanas as explained in the pragmatic dharma movement?

how does it come about and how does it relate to the buddhist practice

Saṅkhitta Dhamma Sutta or The Discourse on the Dharma in Brief mentions you should develop the Jhana factors on the 4 Sathipattana and 4 Brahmavihara.

everyday life?

In daily interaction you create fabrication, but if you keep your focus on the 4 Sathipattana and 4 Brahmavihara with emphasis on arising and passing of feeling (or phenomena which is felt), you can stop or tame the creation of new fabrication hence becoming.

I think the questions you ask are important and very broad. You might consider splitting them up or keeping only the first question "What exactly is Jhana?".

I will point you to some of the litterature where you can find answers on your own.

Here its important to couple the studying with practice or else you will not really understand what Jhana is. You will only understand it intellectually which is like only being able to peek through the door while practicing insight meditation is like opening the door and walking into the room.

Or like the onion-simile. Intellectual knowledge will only allow you to penetrate the top layer while experiental knowledge gained through insight meditation will allow you to penetrate into the deepest layers and the core.

I suggest you check out;

The Visuddhimagga on p. 131-162 the Jhanas are treated.

The Jhanas and the Lay Disciple According to the Pali Suttas by Ven. Bhikkhu Bodhi. Here Bhikkhu Bodhi goes through the Jhanas, Right Concentration, Stream Entry and other important stuff related to Jhanas.

Meditation lecture by Bhikkhu Bodhi. Here Bhikkhu Bodhi talks about both Vipassana & Samatha meditation and explains the 5 hindrances and the Jhanas. I think this lecture is very good and any meditator would learn a lot by listening to this talk.

A Critical Analysis of the Jhanas by Henepola Gunaratana. Here is an analytical study of the Jhanas. It goes through the hindrances and the Jhanas.

Lanka

Jhāna is the Middle-Indian form of Sanskrit dhyāna, which is a simple derivation from the verbal root dhyā or dhyai, meaning "to think of, contemplate, recollect".

In early Buddhism there seem to be two contending theories of the path of enlightenment. The first one is the noble eightfold path, which is quite well known, I suppose. The second is a more technical instruction on meditative stages. The first four of these stages are called the four dhyānas or jhānas. These stages can be found in DN 2 and 3, as well as in MN 27 and 38:

"Having abandoned these five hindrances — imperfections of awareness that weaken discernment — then, quite withdrawn from sensual pleasures, withdrawn from unskillful mental qualities, he enters and remains in the first jhana: rapture and pleasure born from withdrawal, accompanied by directed thought and evaluation.

"This, brahman, is called a footprint of the Tathagata, a scratch mark of the Tathagata, a tusk slash of the Tathagata, but a disciple of the noble ones would not yet come to the conclusion, 'The Blessed One is rightly self-awakened; the Dhamma is well-taught by the Blessed One; the Sangha of the Blessed One's disciples has practiced rightly.'

"Then, with the stilling of directed thoughts & evaluations, he enters and remains in the second jhana: rapture and pleasure born of composure, one-pointedness of awareness free from directed thought and evaluation — internal assurance.

"This, too, is called a footprint of the Tathagata, a scratch mark of the Tathagata, a tusk slash of the Tathagata, but a disciple of the noble ones would not yet come to the conclusion, 'The Blessed One is rightly self-awakened; the Dhamma is well-taught by the Blessed One; the Sangha of the Blessed One's disciples has practiced rightly.'

"Then, with the fading of rapture, he remains equanimous, mindful, & alert, and senses pleasure with the body. He enters & remains in the third jhana, of which the Noble Ones declare, 'Equanimous & mindful, he has a pleasant abiding.'

"This, too, is called a footprint of the Tathagata, a scratch mark of the Tathagata, a tusk slash of the Tathagata, but a disciple of the noble ones would not yet come to the conclusion, 'The Blessed One is rightly self-awakened; the Dhamma is well-taught by the Blessed One; the Sangha of the Blessed One's disciples has practiced rightly.'

"Then, with the abandoning of pleasure and pain — as with the earlier disappearance of elation and distress — he enters and remains in the fourth jhana: purity of equanimity and mindfulness, neither-pleasure-nor-pain.

"This, too, is called a footprint of the Tathagata, a scratch mark of the Tathagata, a tusk slash of the Tathagata, but a disciple of the noble ones would not yet come to the conclusion, 'The Blessed One is rightly self-awakened; the Dhamma is well-taught by the Blessed One; the Sangha of the Blessed One's disciples has practiced rightly.' (MN 27, http://www.accesstoinsight.org/tipitaka/mn/mn.027.than.html)

These four stages are equated with the eigth element of the Noble Eightfold Path, Samyaksamādhi; they are the direct prerequisite for the liberating insight. The texts go on to explain that with this state of mind, the Tathagata-to-be then consecutively directs his mind to his former existences, passing away and arising of beings, the knowledge of the destuction of impurities, whereby finally he attains the realization of the four noble truths and is freed from further births.

Four more meditative "ranges" are prominent in the scriptures, these are

1. ākāśānantyāyatana - the range of infinite space
2. vijñānānantyāyatana - the range of infinite consciousness
3. akiñcanyāyatana - the range of nothing
4. naivasaṃjñānāsaṃjñāyatana - the range of neither ideation nor non-ideation

These meditative "places" were obviously taught by the upanishadic teachers of the Buddha and of ascetics of the time of the Buddha. The way into these is from the corresponding (first, second, ...) jhāna. (cp. MN 8, should also be there in DN 1)

Jhāna, in Theravada Buddhism, refers to the act of meditation:

The (popular etym -- ) expln of jhāna is given by Bdhgh at Vism 150 as follows: "ārammaṇ' ûpanijjhānato paccanīka -- jhāpanato vā jhānaŋ," i.e. called jh. from meditation on objects & from burning up anything adverse (PED, p. 286)

According to the Pali commentaries, it can be of two types:

ārammaṇūpanijjhāna - meditation on objects

lakkhanūpanijjhāna - meditation on characteristics

The former refers to tranquility (samatha) meditation which takes a specific, conceptual object as its focus. The latter refers to insight (vipassanā) meditation, which takes ultimate reality as an object, focusing on cultivating understanding of impermanence, suffering, and non-self (the three universal characteristics) as a vehicle to disenchantment.

The topic of the jhānas is discussed in depth and hotly debated around the Buddhist universe. Check out the great jhana debate on dhammawheel.org as an example.

As a side note, I'm not sure if such a broad, open-ended question is really appropriate for this sort of site. Please someone correct me if I'm wrong.

Originally in Hindu yoga and Jainism, Jhana/Dhyana was deliberate thinking on a given topic.

The word seems to share its root with Sanskrit verb "dhyayati" (pronounced JAH-YA-TEE), that supposedly means "to think over", "to imagine", "to visualize".

Jains, the primary contenders for a honorary title of "precursors to Buddhism", used to recognize four types of Dhyana:

• Artta Dhyana -- serious mulling over problems and issues coming from one's duties and responsibilities
• Rudra Dhyana -- cruel mulling over evil plans
• Dharma Dhyana -- rightful mulling over virtuous subjects
• Shukla Dhyana -- pure mulling, non-conceptual concentration.

Apparently, Buddha's first teacher, Arada Kalama, taught Shukla Dhyana through visualizing oneself in the center of progressively empty context (village --> empty field --> empty sky --> nothingness), culminating in a state of thoughtless concentration with no content.

While Buddha's second teacher, Udraka Ramaputta, supposedly taught him a different practice, that of deliberately thinking the opposite thoughts (so-called "antidotes") to the ones the mind was dominated by, the target state being one of perfect ambiguity.

Buddha famously mastered and then rejected both target states, instead proposing his own progression of dhyana themes, targeted at achieving first a disgust of this body with its lowly interests, then, once the poisons of worldly desires were destroyed by the antidote of disgust, at generating a joyful mind (first with discursive support, then directly), followed by a calm-and-positive mind, and finally an unfettered mind that sees things as they are (tatha). He subsequently replaced disgust of this body with mindfulness of breathing as the object of meditation he recommended to entry-level students.

A state of having mastered an ability to freely enter into and maintain the first of these states (joyful mind with discursive support) is called "the first jhana".

A state of having mastered an ability to freely enter into and maintain the second state (joyful mind without discursive support) is called "the second jhana" and so on.

In this case the word "jhana" has a more specialized meaning of "level of mastery of Buddhist meditation" rather than "meditation" in general.