An internet search for the answer to that question leads you down a rabbit-hole the complexities and tangents of which could be the occupation of a lifetime. Poetry is a term that rebuffs all definition. It is a form of literature too multifaceted, too abstract to neatly define. To be sure, many have opinions on the matter, criteria they impose upon the craft, but poetry refuses to be contained by rules and regulations.
In an attempt to define a poem, many cite the elements of sound play that poets oft employ. Rhyme, rhythm, meter, alliteration, simile, onomatopoeia, the list goes on. These are merely the tools, not the craft itself.
Intuitively, you may read a piece of literature and be sure that it fits beneath the blanket terms of either prose or poetry. Alternatively, you may read a piece that has been labeled as one or the other and just as surely feel that the label is mistaken. Some prose is very poetic and some poetry is by design prosaic. The whole art form is so subjective; I feel it would not be inaccurate to say that poetry cannot be defined.
That being said, what follows are four main elements that I personally find intrinsic to poetry, the nearest thing to a definition that I am willing to propose:
“Poetry is when an emotion has found its thought and the thought has found words.”Robert Frost
Poetry bends language to convey emotion. A poem should make you feel.
“Poetry lifts the veil from the hidden beauty of the world, and makes familiar objects be as if they were not familiar.”Percy Bysshe Shelley
Poetry surpasses the literal, disguising layers of meaning within words that say more than what appears on the surface and often require some untangling to understand. A poem should make you think.
“Poetry should surprise by a fine excess and not by singularity—it should strike the reader as a wording of his own highest thoughts, and appear almost a remembrance.”John Keats
In poetry, words are strung together in ways that defy the usual grammatical rules, in ways that take advantage of the lyrical and performative aspects of language. A poem should make you hear.
“Painting is silent poetry, and poetry is painting that speaks.”Plutarch
Poetry uses often sparse but always striking details to conjure up strong images. A poem should make you see.
Being so subjective, it is natural that some poetry will make you shrug and think I don’t even know what that means or even That’s not a poem, while the same piece for someone else will pierce directly to the heart and draw out deep emotion. It is much like that abstract painting on a gallery wall that elicits both awe and scorn. The line between poetry and prose is imaginary and flexible.
It is both as simple and as impossibly complex as this: A poem is a written work that the author considers to be poetry.