1. A Definition of Poetry from The Concise Oxford Dictionary of Literary Terms

Language sung, chanted, spoken, or written according to some pattern of recurrence that emphasizes the relationships between words on the basis of sound as well as sense: this pattern is almost always a rhythm or *METRE, which maybe supplemented by *RHYME or *ALLITERATION or both. The demands of verbal patterning usually make poetry a more condensed medium than *PROSE or everyday speech, often involving variations in *SYNTAX, the use of special words and phrases (*POETIC DICTION) peculiar to poets, and a more frequent and more elaborate use of *FIGURES OF SPEECH, principally *METAPHOR and *SIMILE. All cultures have their poetry, using it for various purposes from sacred ritual to obscene insult, but it is generally employed in those utterances and writings that call for heightened intensity of emotion, dignity of expression, or subtlety of meditation. Poetry is valued for combining pleasures of sound with freshness of ideas, whether these be solemn or comical. Some critics make an evaluative distinction between poetry, which is elevated or inspired, and *VERSE, which is merely clever or mechanical. The three major categories of poetry are *NARRATIVE, dramatic, and *LYRIC, the last being the most extensive.

2. Poetic Diction from The Glossary of Literary Terms by M. H. Abrams

The term diction signifies the types of words, phrases, and sentence structures, and sometimes also of figurative language, that constitute any work of literature. A writer's diction can be analyzed under a great variety of categories, such as the degree to which the vocabulary and phrasing is abstract or concrete, Latin or Anglo-Saxon in origin, colloquial or formal, technical or common. See style and poetic license.

Many poets in all ages have used a distinctive language, a "poetic diction," which includes words, phrasing, and figures not current in the ordinary discourse of the time. In modern discussion, however, the term poetic diction is applied especially to poets who, like Edmund Spenser in the Elizabethan age or G. M. Hopkins in the Victorian age, deliberately employed a diction that deviated markedly not only from common speech, but even from the writings of other poets of their era. And in a frequent use, "poetic diction" denotes the special style developed by neoclassic writers of the eighteenth century. This neoclassic poetic diction was in large part derived from the characteristic usage of admired earlier poets such as the Roman Virgil, Edmund Spenser, and John Milton, and was based on the reigning principle of decorum, according to which a poet must adapt the "level" and type of his diction to the mode and status of a particular genre see style). 

But what were ranked as the higher genres, such as epic, tragedy, and ode, required a refined and elevated poetic diction to raise the style to the level of the form, while pastoral and descriptive poems, which necessitated reference to what were considered lowly materials, used a special diction to invest these materials with a dignity and elegance appropriate to poetry. Prominent characteristics of eighteenth-century poetic diction were its archaism and its use of recurrent epithets; its preference for resounding words derived from Latin ("refulgent," "irriguous," "umbrageous"); the frequent invocations to, and personifications of, abstractions and inanimate objects; and the persistent use of periphrasis (a roundabout, elaborate way of saying something) to avoid what were perceived as low, technical, or commonplace terms by means of a substitute phrase that was thought to be of higher dignity and decorum. 

In William Wordsworth's famed attack on the neoclassic doctrine of a special language for poetry, in his preface of 1800 to Lyrical Ballads, he claimed that there is no "essential difference between the language of prose and metrical composition"; decried the poetic diction of eighteenth-century writers as

"artificial," "vicious," and "unnatural"; set up as the criterion for a valid poetic language that it be, not a matter of artful contrivance, but the "spontaneous overflow of powerful feelings"; and, by a drastic reversal of the class-hierarchy of linguistic decorum, claimed that the best model for the natural expression of feeling is not upper-class speech, but the speech of "humble and rustic life."