This is peace, this is exquisite — the stilling of all fabrications, the relinquishment of all acquisitions, the ending of craving, dispassion, cessation, Unbinding (Nibbana). (MN 64)
In the Mulapariyaya Sutta (MN 1) the Buddha advises us not to conceptualize about Nibbana as a philosophical endeavor. Regarding such speculation, he states:
There is the case, monks, where an uninstructed run-of-the-mill person — who has no regard for noble ones, is not well-versed or disciplined in their Dhamma; who has no regard for men of integrity, is not well-versed or disciplined in their Dhamma ... perceives Unbinding as Unbinding. Perceiving Unbinding as Unbinding, he conceives things about Unbinding, he conceives things in Unbinding, he conceives things coming out of Unbinding, he conceives Unbinding as 'mine,' he delights in Unbinding. Why is that? Because he has not comprehended it, I tell you.
But for the sincere aspirant, he states that we should come to know Nibbana directly, through letting go of all conceptual speculation:
He directly knows Unbinding as Unbinding. Directly knowing Unbinding as Unbinding, let him not conceive things about Unbinding, let him not conceive things in Unbinding, let him not conceive things coming out of Unbinding, let him not conceive Unbinding as 'mine,' let him not delight in Unbinding. Why is that? So that he may comprehend it, I tell you.
And he goes on to say that this is exactly how the arahants (enlightened ones) regard Nibbana. Elsewhere, to give us some orientation toward where we should head, the Buddha gives us various adjectives and synonyms for Nibbana, such as:
The unfashioned ..., the end ..., the effluent-less ..., the true ..., the beyond ..., the subtle ..., the very-hard-to-see ..., the ageless ..., permanence ..., the undecaying ..., the featureless ..., nondifferentiation ..., peace ..., the deathless ..., the exquisite ..., bliss ..., solace ..., the exhaustion of craving ..., the wonderful ..., the marvelous ..., the secure ..., security ..., Nibbana ..., the unafflicted ..., the passionless ..., the pure ..., release ..., non-attachment ..., the island ..., shelter ..., harbour ..., refuge ..., the ultimate. (Samyutta Nikaya 43)
Now it's clear from terms like: unmade (akata); unconditioned (asankhata); uncreated (asankhara); endless (ananta); indestructible (apalokita); permanent (dhuva); deathless (amata); etc., that Nibbana is unconditioned and permanent. It is the permanent cessation of suffering (dukkha nirodha) brought about by the cessation of craving (tanha) and the cessation of craving's root cause, ignorance (avijja). This cessation of suffering, craving, and ignorance is what the Buddha is ultimately saying about this/that conditionality when he states in the Bodhi Sutta (Ud 1.3):
When this isn't, that isn't.
From the cessation of this comes the cessation of that.
And when craving and ignorance are brought to permanent cessation one is unconditionally and permanently free from all future suffering because there is 'release' (vimutti) from ignorant reification of temporality as well as the inevitable becoming, birth, and death resulting from that ignorance and craving. One is released from the twelvefold cycle of dependent origination. As the Buddha says in the Upaya Sutta (SN 22.53):
[O]wing to the abandonment of passion, the support is cut off, and there is no base for consciousness. Consciousness, thus unestablished, not proliferating, not performing any function, is released. Owing to its release, it is steady. Owing to its steadiness, it is contented. Owing to its contentment, it is not agitated. Not agitated, he is totally unbound right within. He discerns that 'Birth is ended, the holy life fulfilled, the task done. There is nothing further for this world.'
By abandoning passion (lobha, raga, tanha, etc.) toward the conditioned body and mind through the proper development (bhavana) of calm abiding (samatha) and clear seeing (vipassana), sensory consciousness and the other four conditioned aggregates are released. This is discernment-release (panna-vimutti), wherein one first comes to a full experiential understanding of all conditioned phenomena (dependent origination, impermanence, suffering, and not-self). This understanding is termed 'gnosis of the regularity of phenomena' (dhamma-thiti-nana), and this direct knowing of conditioned phenomena culminates in the cessation of ignorance with the discernment that 'Birth is ended, the holy life fulfilled, the task done. There is nothing further for this world.' This discernment is termed 'gnosis of Nibbana' (Nibbana-nana), which is the fruition of discernment release. This is stated by the Buddha in the Susima Sutta (SN 12.70):
First, Susima, there is the knowledge (gnosis) of the regularity of the Dhamma [dependent co-arising], after which there is the knowledge of Unbinding.