"I don't think a person without a love of nonsense could become a poet. The nonsense is there in the very soul of language. All that reaching after meaning, all those beautiful sounds, all that magnificent failure." (George Szirtes)
That moment in poems that cannot be reduced to sense because it is not motivated by sense: whether rhyme, meter, word choice, or some Oulipian constraint. Not necessarily the nonsense verse that Szirtes refers to, but something playful even in the most serious verse, "something there is that doesn't love a wall," as it were.
German speakers could take a look at Rüdiger Görner's article on W. H. Auden in the Neue Zürcher Zeitung of February 17. Görner claims that one can never be sure that Auden did not say things just for the sake of rhymes, such as:
Earned a meagre salary,
Walking through the Bois,
Observing his Moi.