That's a great question. I ask it of myself frequently.
Even though I don't have a firm answer, I've continued on with using the words "spiritual" and "spirituality" a lot – in talking with people, in writing on this blog, in musings within my own mind.
I used to think that spiritual was opposed to material, that it had something to do with an ethereal soul, an aspect of ourselves that is disconnected (potentially, at least) from physical reality.
But this would mean that you'd have to leave this world to be genuinely spiritual. Since everyone on Earth is, obviously, earthly, spirituality would be impossible here.
Reading Andre Comte-Sponville's "The Little Book of Atheist Spirituality," I came across an understanding of "spiritual" that makes sense to me. More sense, in fact, that the sense I was speaking to myself.
Spirituality is the life of the spirit. But what is the spirit? "A thing that thinks," said Descartes, "that is to say, that doubts, affirms, denies, that knows a few things, that is ignorant of many, that wills, that desires, that also imagines and perceives."
And I would add: a thing that loves, that does not love, that contemplates, that remembers, that mocks or jokes…Little does it matter whether the thing in question is the brain, as I believe it to be, or an immaterial substance, as was Descartes's conviction. Whatever it is, we use it to think, to want, and to imagine.
…The spirit is not a substance. Rather, it is a function, a capacity, an act (the act of thinking, willing, imagining, making wisecracks…) – and this act, at least, is irrefutable, since nothing can be refuted without it. "The spirit is not a hypothesis," as Alain put it, because hypotheses can exist only by and through the spirit.
In short, it's our inner life. Not something mystical, airy-fairy, mysteriously divine, other worldly. It's what we are at every moment, when we're aware of what we are at every moment (and also, not aware).
Today, during my Tai Chi class. I caught a glimpse of myself in the mirrors that run along one wall of the studio. I don't like to look directly at my image. It creeps me out. Usually I keep my focus close to the bottom of the mirrors.
Probably that isn't good for my Tai Chi posture, but I don't care. I'd rather not be reminded for an hour and a half of what the outer Brian looks like, now that he's reached the graying age of 59.
But this afternoon a thought popped into my head: "That's not you." At that instant I found that looking at myself didn't bother me as much. I'd made a disconnect between the inner me and the outer me, the spirit and the body. This is how Comte-Sponville sees things also.
Taken in its broadest sense, spirituality can be said to include virtually all aspects of human life and spiritual is more or less synonymous with "mental" or "psychic." Today, this use of the word has pretty much gone out of use, and when people talk about spirituality, they are usually referring to a rather limited part of our inner life (though it may contemplate limitlessness) – the part that involves the absolute, the infinite and the eternal. It is, in a sense, the spirit's farthermost point and its greatest amplitude.
Spirituality, then, doesn't necessarily entail religion. In fact, there's no necessary connection between them.
We are ephemeral beings who open onto eternity and relative beings who open onto the absolute. This "openness" is the spirit itself. Metaphysics means thinking about these things; spirituality means experiencing them, exercising them, living them.
…This is what distinguishes spirituality from religion, which is merely one of its possible forms. The two can be conflated only by virtue of metonymy or misnomer. They are as whole and part, genus and species. All religions involve spirituality, at least to some extent, but all forms of spirituality are not religious.
Whether or not you believe in God, the supernatural or the sacred, you are confronted with the infinite, the eternal and the absolute – and with yourself. Nature suffices. The truth suffices. Our own transitory finiteness suffices. Otherwise we could not conceive of ourselves as being relative, ephemeral or finite.