Poetic diction is the language, including word choice and syntax, that sets poetry apart from more utilitarian forms of writing.
From A Poet’s Glossary
The following additional definition of the term poetic diction is reprinted from A Poet's Glossary by Edward Hirsch.
Poetic diction refers to the operating language of poetry, language employed in a manner that sets poetry apart from other kinds of speech or writing. It involves the vocabulary, the phrasing, and the grammar considered appropriate and inappropriate to poetry at different times. In Poetic Diction: A Study in Meaning (1928), Owen Barfield writes, “When words are selected and arranged in such a way that their meaning either arouses, or is obviously intended to arouse, aesthetic imagination, the result may be described as poetic diction.”
Aristotle established poetic diction as a subject in the Poetics (350 BCE). “Every word is either current, or strange, or metaphorical, or ornamental, or newly-coined, or lengthened, or contracted, or altered,” he declared, and he then considered each type of word in turn. His overall concern was “how poetry combines elevation of language with perspicuity.” Changes in poetic fashion, reforms in poetry, often have to do with the effectiveness of poetic diction, the magic of language. How, if at all, is poetic speech marked differently than ordinary speech? “The weightiest theoretical legacy which antiquity and the Renaissance passed on to neoclassicism was the ornamental conception of poetic style,” Emerson Marks writes. “Till the dawn of Romanticism, writers continued to regard the characteristics of verse as raiment adorning the ‘body’ of a poet’s thought.” In The Life of Dryden (1779–1781), Samuel Johnson argued that before the time of Dryden, there was simply
no poetical diction: no system of words at once refined from the grossness of domestic use and free from the harshness of terms appropriated to particular arts. Words too familiar, or too remote, defeat the purposes of a poet.
In the preface to Lyrical Ballads (1802), William Wordsworth argued against the ornate effects of his predecessors and insisted on the essential identity of poetic and non-poetic language. He argued that poetry should employ “the real language of men in any situation.” Wordsworth revolutionized the idea of poetic diction by connecting it to speech. Poetry is linked to speech, to the way that people actually talk at any given time, but it is also framed and marked differently.