As we all know, all writing is either prose or poetry, so it seems reasonable to say “If it ain’t poetry, it’s prose, and if it ain’t prose, it’s poetry”. So the question of defining poetry is really a matter of distinguishing between the two, and the best distinction I have heard came from my daughter when she was about seven or so: prose, she said – or, rather, “normal writing”, there being nothing quite so abnormal as poetry – goes all the way to the right hand side of the page (except at end of paragraphs); and poems don’t. While this admittedly leaves out of consideration those curious hybrids “prose poems”, neither before nor since have I heard the distinction between prose and poetry laid out quite so clearly.
For prose is written in units of sentences which may cut across lines, whereas poetry is written in units of lines which may cut across sentences. Of course, this leaves open the question of why one should wish to write in units of lines rather than that of sentences, but that consideration, important though it may be, is outside the scope of defining poetry: as far as mere definition goes, poetry has, I think, been well and truly defined.
And it provides an apt answer to those who insist that mere prose broken up more or less at random into lines falling short of the right hand margin “isn’t poetry”; or those who tell us that mere banal doggerel cannot be poetry either. As far as I’m concerned, if the author says it’s a poem, then, goddammit, it’s a poem. Of course, whether or not it’s a good poem is another matter entirely.