No one dares speak out against "word casing" sacred cows like "enlightenment," yet few understand them, not empirically, at least. It would be much better to use simpler language, metaphysical in place of spiritual, for instance, because metaphysical is a term most people can agree on. It means 'beyond the physical' while spiritual connotes a more personal meaning for most.
In fact, before my presentations, I hand out pieces of paper to the audience, asking them to write down a one-sentence definition of the word spiritual. You know what I get? Forty or fifty completely different, largely contradictory definitions. It's not the same with metaphysical; people easily agree on its meaning.
Since the understanding of terms like 'enlightenment' and 'duality' are based on our experiences in the physical world, it is only normal that our 'definitions' reflect our material perceptions and experience. In other words, the boundaries of our material concepts and our entrapment therein.
After a Kundalini awakening, we begin to experience beyond-the-physical, meta effects for the first time in most instances. This provides us with kind of 'grokking' insight into the beyond-the-physical realm. All of a sudden we know there is no death, no duality. This realization is empirical, a product of Kundalini awakening. Over time we move gradually from the physical to a greater understanding of the metaphysical, but it takes time, lots of time. At the same time, we are able to apply the same observational, scientific principles to our metaphysical experiences as a scientist applies to a laboratory experiment.
If you're truly interested in exploring the metaphysical, forget about Terminology and practice.
"Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great." ~Mark Twain