As writers we are familiar with the anatomy of a story, the little pieces that make the whole. From adverbs to tense, plot to pacing, we can point to the word on any given page and say - "There it is, I see it." All plots can be reduced to a witch's hat; a physical manifestation of the course of the story. The bones and muscles, sinews and capillaries that compose the body of our story exist in a very real sense.
So what about the soul?
That's what I call "voice."
And if you find me an accurate diagram of a soul, make sure you tell the Pope too. Chances are, he'll be interested.
Voice is the ephemeral personality of the story, whether it be cocky attitude or wistful pondering. Voice is the single most difficult variable in writing, the make or break that's going to connect your reader or turn them off like a radio dropped in the toilet. And it's impossible to explain.
I was at a conference where a panel of agents and editors were asked by an audience member, "How do you define voice? And please don't tell me you know it when you see it." An agent promptly took the mic and said, "I know it when I see it."
It's like asking someone - "Why do you like your friends?" Uh well, she's nice, and uh, funny, and... well, she's... my friend. Can you reduce your friends to adjectives? No. You connected with them because of their personality, because their internal, intangible voice said something to your own - and you began a conversation.
One more metaphor as I attempt to define the impossible.
If you look at a diagram of a flower, you won't see smell labeled.
Yet what do we think of when we think of a rose? Or a hyacinth? Or heck, even a big fat pile of dog crap? Smell. And if you pick any of those things apart, you're not going to find "smell" in there. It radiates from the whole but can't be singled out.
And so with voice.