Poetic Diction was a highly artificial, stilted and unnatural mode of writing used by the Neo-classical poets in writing their poetry. They took pride in using highly obscure, unfamiliar, quaint and high-sounding words and expressions which are hardly ever used in day-to-day life. By using such words and expressions they sought to show off their highly scholastic status and superiority. Wordsworth exploded this vanity of the Neo-classicists in the preface to the Lyrical Ballads. He decries the poets who think that "they are conferring honour upon themselves and their art in proportions as they separate themselves from the sympathies of men and indulge in arbitrary and capricious habits of expression."
Explaining his point of view, Wordsworth says that poets do not write for poets alone, but for men. A poet must express himself as other men express themselves. The poet should imitate, and as far as possible, adopt the very language of men. The expressions used in Poetic Diction do not make any natural or regular part of that language. A poet must bring his language as far as possible near to the language of men. There neither is, nor can ever be, any essential difference between the language of prose and metrical composition.
The language of a large portion of every good poem in no respect differs from that good prose. Therefore Poetic Diction is cumbersome artifice which must be abandoned.