Is nibbana eternal?

I think that sankharas arise and cease -- they arise dependent on other things, created from or of other things, when and only when other things exist -- and they may cease when those conditions (dependencies, components) change or cease.

Conversely nibbana is defined as "unconditioned", and therefore isn't a sankhara (it's a dhamma), and isn't siubject to arising and ceasing -- it's not impermanent, not anicca.

I don't know what the doctrine says about the word "eternal" -- it says that "the dhamma" is akalika (meaning "timeless" or "without time", possibly "immediate" or "ever-present", I don't know about "eternal"). I think that "dhamma" used in that sense (to refer to Buddhist doctrine) isn't necessarily the same as saying that nibbana is a "dhamma" rather than a "sankhara" -- apparently "dhamma" is a word with several meanings/uses.

In SN 56.11 it's not "dhamma" that arises but rather dhammacakkhu or "insight into dhamma" which is associated with "stream entry".

Whether that is eternal is another matter.

Think carefully the conditioned (arising dhamma) comes out of the unconditioned (nibbana). Without the unconditioned there is no conditioned either.

You might find doctrine like that in Mahayana schools, e.g. (I don't know) Nagarjuna.

Also some schools of Buddhism (e.g. Tibetan) say that nibbana is temporary too, though apparently that sounds contrary to e.g. the doctrine of the Pali suttas.

For Mahayana doctrine you might want to look at topics such as:

• How is samsara nirvana?
• Difference between abiding nirvana and non-abiding nirvana?

I think it's contrary to Theravada doctrine, though -- perhaps because Theravada defines nibbana as a state in which defilements and so on don't ever arise, will no longer ever arise, never arise, cannot arise, in which the conditions for their arising don't exist (like a palm tree which has been cut down doesn't regrow).

There's some complicated doctrine though about whether nibbana is a state of mind, based on whether how it's perceived by an arahant -- see e.g. this footnote:

Some have objected to the equation of this consciousness with nibbana, on the grounds that nibbana is nowhere else in the Canon described as a form of consciousness. Thus they have proposed that consciousness without surface be regarded as an arahant's consciousness of nibbana in meditative experience, and not nibbana itself.

I'm not sure whether or why that's a doctrinal distinction worth making, though.

The pali is the usual “yaṃ kiñci samudayadhammaṃ sabbaṃ taṃ nirodhadhamman”ti, which is the usual, badly said: whatever will samudaya will nirodha. so it is the usual whatever is subject to arising will cease too.

And nibanna does not arise and does not cease, that's the whole point of nibanna.

'Lasting forever' implies relation to time. Nibbana is not related to time!

Nibbana is a state of unconditioned reality, aka Ultimate Reality. Therefore it cannot be measured or quantified, making Nibbana neither eternal vs temporal, neither permanent vs impermanent, neither long vs short, neither existent vs non-existent, neither when & where...etc, because such concepts applies only to conditioned reality, aka Conventional Reality, which is where we are now.

"Everything that has a beginning has an end.” means every-thing, every conditioned, manifested thing is impermanent. Nibbana is not conditioned and it is not manifested state, so it does not have a beginning or end

My English is not good so I looked to the meaning of eternal. It says that: "lasting or existing forever; without end"

If you try to describe Nibbana with using "time" then it won't lead you to the right direction because Nibbana is the unconditioned. It is timeless. You can use that term, If you mean being "deathless" by saying eternal. Nibbana is free from any conditioned state. It is free from birth, death, time. It is the unmanifested state.

Thich Nhat Hanh wonderfully talks about Nibbana: https://www.youtube.com/watch?v=odWIPhj-ivo

What we usually understand as happiness is dependent on specific condition, which need to be controlled, arranged, fulfilled and satisfied in order to reach that specific and temporary state.

The happiness of Nibbana is what remains after removing the desire for control and satisfaction of the senses and the ego. If you eliminate the conditions for suffering, what remains is peace without conditions. Thus, you don't need anything in special to feel satisfied, because craving, aversion and ignorance have been eradicated. You feel content because you took off what blocked that peace of simplicity and renunciation.

That's why it's called "The Unconditioned".

As long as the conditions (craving, aversion and delusion) are uprooted, Nibbana should be eternal. If those conditions are still there in the mind, then, at most, you can have "glimpses" of Nibbana.

Also, remember that: "Sabbe sankhara anicca": everything conditioned is impermanent.

"Sabbe sankhara dukkha": everything conditioned is unsatisfactory.

"Sabbe dhamma anatta": all dhammas are not-self

The word "Dhamma" can be used to talk about the Buddha's teachings, as well to refer to any phenomena. It all depends on the context. And I think the quote you posted is talking about conditioned dhammas, i.e. sankharas.

Kind regards!