There is no one answer, and many, many people have their own ideas.
Here are some definitions we like:
‘Literary work in which the expression of feelings and ideas is given intensity by the use of distinctive style and rhythm’ – OED
‘Poetry is a heightened imagistic use of language that does things to the heart and head’ – Grace Nichols
‘A made thing, a verbal construct, an event in language’ – Edward Hirsch
‘Poetry is memorable speech’ – W H Auden
‘Poetry does not explain life. It gives life to feeling and seeing’ – Sarah Stetie
‘Why, Sir, it is much easier to say what it is not’ – Samuel Johnson