Poetry is a subset of literature, the art form of language, but it also legitimately belongs to another art: music.
Poetry is rhyme and rhythm. It is cadence and count, meter and metric. Poetry is prosody. It is scansion. It is versification. And those are the elements of poetry that make it a part of the musical.
But poetry is primarily a branch of literature, and the two main elements of poetry are style and theme. (There is such a thing as narrative poetry, which is poetry that tells a story, but those two elements — storytelling and verse — combine rather poorly.)
It’s important to point out that the word “poetry” is not synonymous with the word “poem.”
Poetry is general; poems are specific.
All poems are in theory poetic, but not all poetry is a poem.
Novels, essays, memoirs, chronicles, short stories, and virtually every other form of prose can be poetic. For example, “The multitudinous seas incarnadine” is a poetic passage, but it’s not a poem.
A poem, by definition, is a self-contained piece, of varying length, with a certain meter, rhythm, and style, all of which combine to convey a theme. A poem can rhyme or not.
The definition of poetry, on the other hand, has confounded writers and philosophers for centuries. Leo Tolstoy captured this well when he wrote:
Where the boundary between prose and poetry lies I shall never be able to understand. The question is raised in manuals of style, yet the answer to it lies beyond me. Poetry is verse: prose is not verse. Or else poetry is everything with the exception of business documents and school books.
But even “business documents and school books” could — at least, in theory — be poetic.
So what is poetry?
Poetry is style: stylized language.
Poetry is concentrated speech. It is density of expression.
Poetry is language at its best.
Poetry is writer’s writing.
Poetry is not, contrary to popular belief, pretentious or flowery language — or, at any rate, good poetry is not.
Poetry is technique. Poetry is skill. Poetry is metaphor.
Poetry is the beauty of language.