Dhyana is the 7th of the eight limbs of yoga and it means meditative absorption. The Pali word used in Buddhism to describe this state is Jhana.
As we practice dharana or single pointed-concentration, periods of sustained focus, uninterrupted by distraction, increase. The senses become quiet. Only the mind is active, submerged in a deep stream of stable consciousness. Narrative, memory, and ego fall away and only awareness of being and object remain. This continuous flow of concentration is called dhyana.
Meditation is sustained concentration, whereby the attention continues to hold or repeat the same object or place. – Yoga Sutra 3.2
Dhyana is deeper than the vigilant maintenance of concentration in dharana and insight arises resulting in increased self-knowledge. However, dhyana naturally occurs with the repeated practice of dharana. Over time, discursive thought, stories and concepts fade into the background. A feeling of rapture or joy arises, evolving into a state of equanimity and a sense of boundlessness. Through the clear seeing and knowing resulting from dhyana, we begin to get a glimpse of our true nature – pure consciousness, limitless and fathomless.
And what is Right Concentration? …secluded from sense desires, secluded from unwholesome states of mind… enters and remains in the First Jhana (dhyana) which is filled with rapture and joy born of seclusion accompanied by initial and sustained attention. With the stilling of initial and sustained attention, by gaining inner tranquillity and oneness of mind, he enters and remains in the Second Jhana (dhyana) which is without initial and sustained attention; born of concentration, and is filled with rapture and joy. With the fading away of rapture, remaining imperturbable, mindful, and clearly aware, he enters and remains in the Third Jhana (dhyana), and of him the Noble Ones declare, “Equanimous and mindful, he has a pleasurable abiding.” With the the abandoning of pleasure and pain — as with the earlier disappearance of elation and distress — he enters and remains in the Fourth Jhana (dhyana): which is beyond pleasure and pain; and purified by equanimity and mindfulness. This is called Right Concentration. – Mahasatipatthana Sutta