What exactly is Jhana? What exactly is the theory of jhana, how does it come about and how does it relate to the buddhist practice and everyday life?

feel free to answer in depth.
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.