I enjoy arguing about possible definitions of poetry - well I enjoy arguing anyway but it's especially pleasurable to argue about something which is as difficult to define as the above. I usually say something like: The only thing which defines a poem is that the lines don't run to the edge of the page. (I disregard so called prose poems).
I was delighted then when Terry Eagleton in his book How to Read a Poem offers this definition: A poem is a fictional, verbally inventive moral statement in which it is the author, rather than the printer or word processor, who decides where the lines should end.
I'm enjoying the book. It's a wide ranging and comprehensive look at poetry criticism which the author says seems to be something of a dying art. It's also highly entertaining with quite a number of Irish references as you would expect from Eagleton. Yeats figures prominently in it as well as Mahon and Heaney and the author even has a swipe at the way Van Morrison uses sudden snatches of passion and heartfelt breathing to try to enhance his singing of old Irish songs.
Terry Eagleton in the Times online on how to read a poem here.