The word poem arrives in English from the late 15th century, by way of the Latin poema which itself comes from Greek poiēma, that derives from the earlier term poiein, meaning 'create.'
 
But what is a poem? To be honest, I've never found a very good formal definition. The best I've come across went something like, "a collection of words that express an idea, somtimes containing specific rhythm." That works but it also sucks. Too vague. I once asked a group of biology nerds at a college party for a definition of poem. Folks mostly replied with references to other forms of expression, such as "a poem is a form of painting with words" or "a song without music accompanying it." All of these were interesting. But the response I liked most was, "a creative written or verbal device employed in emergency situations." I laughed at that for about an hour.