My own definitions of poetry when asked in class:
1. Poetry is a series of words and phrases arranged with the intention of attaining aesthetic and mnemotechnic effects; in other words, the arrangement needs to be "beautiful" (whatever it may mean) and memorable.
2. Poetry is a set of words charged with condensed possibilities of meaning beyond a simple, linear communication.
Then Jen gave us some examples given by poets of which I quote the following:
Poetry is---
"memorable speech" (Auden) [Did I know this from somewhere?]
"a machine made of words" (William Carlos Williams)
"an emotional machine made of words" (Phillip Larkin)
"the synthesis of hyacinths and biscuits" (Carl Sandburg)