When a poem is finished, you can't move anything around in it -- you can't substitute one word for another, you can't change a punctuation mark or a line division or anything sort of phrasing without diminishing the effect to some degree. So in other words, it's language brought to a kind of state of perfection. Obviously there is no such thing as perfection, but as close to it as possible. I think that's one of the tests. Most prose paragraphs you can move things around without diminishing the effects, but a poem--everything is in its place.
-- Ted Kooser --
Have a great Wednesday!