Here is the problem. There is the nihilistic marketplace of spiritual materialsim as it has been rightly called. There are so many “paths” that it is impossible to know which one to take, whether they are the same or different in essential ways and whether they lead to same or different goals, and how to choose between them. They all make claims to take you to “enlightenment”. So you are going to get different answers from different practitioners, and it is going to be difficult to adjudicate their claims.
However, when we speak of “enlightenment” what normally comes to mind is some form of Buddhism. So here we will narrow our focus to Buddhism.
What is interesting is that Buddhism is a particular response to the Indo-european worldview as it was expressed in India as Hinduism. What is not normally discussed is how Buddhism has isomorphies to the Indo-european worldview in many interesting ways. The Indo-european worldview based on Indo-european langages have the unique property of having Being as the central concept in the language. All other language families are existence based, or perhaps only have the copula. Thus Buddhism can be seen as a radical rebellion against this inherent perspective of the Indo-europeans through their langage and an attempt to return to an understanding of Existence, which is beneath the projections of the illusions of Being (Sat). Buddhism was a radical heresy with respect to Hinduism because it called into question the whole idea of the perdurance of Being and substituted the idea of “emptiness”. Emptiness is defined in terms of the tetralemma (A, ~A, Both, Neither). Emptiness is a non-concept, non-experience that goes beyond all the logical permutations. It is thought to go back to the Buddha himself but its use was perfected by Nagarguna who showed that logic itself had emptiness in it, because of the essential fragmentation between its operators (and, or, nand, nor). It is interesting that Aristotle specifically calls out the Tetralemma in his Metaphysics as the greatest mistake, and produces noncontradiction and excluded middle to combat this fundamental error. Thus the Tetralemma has an important place in founding the dualism of the Western tradition. There are other examples of traditions that have given rise to nondual approaches. There is the void of Taoism, and there is also the heresy of Islam later in the West.
So formally “emptiness” is what is other than the elements of the tetralemma. Thus Nagarjuna will take any distinction and critique all four positions without giving a positive answer. He is like the Buddha silent when asked whether God existed. This has a lot of parallels to the Skepticism of Sextus Empiricus which was the strawman that was attacked by everyone within the Western tradition until Nihilism took its place. Actually I think that nihilism took the place of skepticism when the West became aware of Buddhism. Because of the fact that Being in the West had pride of place as the central concept in the tradition everyone was shocked by the idea that people could advocate the emptiness of everything. Hegel eventually placed this distinction at the base of his dialectical logic. The only way to absorb it was to make the distinction between Being and Nothing the most fundamental distinction from which everything else dialectically proceeded. For Hegel, the two together produced the flux of Heraclitian change and suddenly philosophers had to deal with history, rather than only proposing ahistorical doctrines.
It is important to note that emptiness is itself empty, so we cannot reify it in though or experience.
The Buddha was addressing the fundamental problem in the Hindu tradition which was Karma. He posited that there was an escape from the infinite pressure of Karma though what is called Nirvana, which was obliteration of the Self and ceasing of the wheel of Karma for the individual soul. Emptiness was the logical Other to Karma which was the working out of all the permutations of things in the world in terms of lives lived.
The fact that Buddhism accepted Karma and emptiness both served as one of the motive forces for the development of Buddhism because these two ideas are hard to reconcile. The question eventually became if everything is empty how can there be karmic traces (bija) if they are themselves empty and there is no were to lay down those seeds of subsequent incarnations.
In Buddhism the trick is to realize the nature of emptiness and that is what leads directly to enlightenment. The nature of emptiness is that it is itself empty, and thus it does not exist. But it is a way to view existence and to wipe away the illusions of maya, mara, dukkah that are produced by the concept of Being (Sat). The Self is asat. That is the fundamental proposition of Buddhism from the beginning. Eventually all the Dharmas are seen to be empty as well. Existence is seen to be aggregates in constant flux more or less as Heraclitus said.
But as Buddhism developed the Buddha kept coming up with more and more interesting things to say about enlightenment. Sutras kept turning up and the teaching evolved, as practitioners got more experience meditating, and reaching different states, and so there is a whole history of the development of the schools of Buddhism where enlightenment is continually being reconceived.
But I think for us the fact that Buddhism is a heresy that says that Being does not exist is the most interesting thing. It applies to the Western worldview as it did to the Hindus when Buddhism arose until it was reabsorbed into Hinduism with the Buddha becoming one of the avatars of Vishnu like Krishna.
From the beginning with Aristotle’s Metaphysics the Western worldview has been dualistic and anti-nondual. When a nondual heresy would spring up the Westerners would do everything they could to stamp it out. But to be fair they would also stamp out extreme dualists as well. But there are very few examples of nondual positions within the Western tradition. My favorite is Meister Eckhart but if he had not died before the inquisition could get to him he would have been treated like the other heretics in the Westen tradition who supported nondual doctrines. And our tradition is just as dualistic today. But that inherent dualism calls produces a yearning for something else, which various nondual traditions, like Buddhism, Taoism, Sufism and other paths offer in various ways.
Buddhism attacks the core of the Western worldview which is the production of nihilism from out of the fragmentation of Being by turning nihilism back on itself so that it falls on its own sword so Being self-destructs and one gets a glimpse of the bedrock of Existence beneath the illusion of Being. Existence is interpreted as empty in Buddhism, and it is interpreted as void in Taoism. Either way existence is wajud (i.e. what is found) in Arabic. It is like the rock at the side of the road that no one cares about. It is just there and no one cares about it. Being is when we start projecting on those rocks some a priori transcendentals.
So to sum up the nature of enlightenment is the realization by some means the true and real nature of existence beneath the projected illusions (mara, maya, dukkah, etc). One way to do that is to invest everything in the pursuit of enlightenment and then to suddenly find it does not exist. But another way is to just look at what is around you without investing anything in it, i.e. with non-attachement. Suddenly perhaps you will see what has always been there by you never noticed before.