Inspired by the Oregon Shakespeare Festival's laudable decision to colorize—um I mean translate Shakespeare into a more user-friendly demotic, I thought I'd just update Robert Frost's germane and helpful definition:
I could define poetry this way: it is that which is lost out of both prose and verse in translation.
To clarify for a modern audience, the way they're doing it at the OSF, with their Shakespeare knock-offs:*
One operational definition of poetry, whether you are referencing officially sanctioned poetry in a recognizable verse form or, more controversially, the elements of prose that impact us perceptually as "feeling like poetry feels" (as one could informally put it) might be usefully analogized as an as yet unsolved and perhaps unsolvable problem for machine translation.
*Shakespeare knock-off, thanks to one contemporary translator who helps us with this update of a line of Macbeth's: "The deep damnation of his knocking-off."
Remember, Scotland runs on Duncan.