It is a key concept in Hinduism and Buddhism. According to the Hindu Yoga Sutra dhyana is one of the eight methods of Yoga, (the other seven methods are Yama, Niyama, Asana, Pranayama, Pratyahara, Dharana, and Samadhi). Theravada Buddhism recognizes eight progressive states of dhyana.[1] (http://www.geocities.com/Tokyo/6774/jhana2.htm). In East Asia, several schools of Buddhism were founded that focused on dhyana, under the names Chan, Zen, and Seon. According to tradition, Bodhidharma brought Dhyana to the Shaolin temple in China, through Tibet, where it came to be known first as ch'an, and then zen.
Dharana is the preceding stage of Dhyana. In Dhyana, the meditator is not conscious of the act of meditation (i.e. is not aware that s/he is meditating) but is only aware that s/he exists (consciousness of being), and aware of the object of meditation. Dhyana is distinct from Dharana in that the meditator becomes one with the object of meditation and is able to maintain this oneness for 144 inhalations and expirations.
The singleness of jhana means not only that awareness is focused on a single object, but also that the object is reduced to a single quality that fills the entirety of one's awareness, at the same time that one's awareness broadens to suffuse the entire object.
Jhana focused on this type of form comes in four levels, identical with the four levels mentioned in the definition of the faculty of concentration [§72] and of right concentration under the noble eightfold path [§102].
These are the four formless jhanas: the sphere of the infinitude of space, the sphere of the infinitude of consciousness, the sphere of nothingness, and the sphere of neither perception nor non-perception.
The sense-sphere (kamadhatu) is the field of rebirth for evil deeds and for meritorious deeds falling short of the jhanas; the fine-material sphere (rupadhatu), the field of rebirth for the fine-material jhanas; and the immaterial sphere (arupadhatu), the field of rebirth for the immaterial jhanas.
In their capacity for producing concentration the jhanas are called the basis (pada) for insight, and that particular jhana a meditator enters and emerges from before commencing his practice of insight is designated his padakajjhana, the basic or foundational jhana.
Thence, taking the absorptive force of the jhana factors as the criterion, the paths and fruits may be reckoned as belonging to either the first, second, third or fourth jhana of the fourfold scheme, or to the first, second, third, fourth or fifth jhana of the fivefold scheme.