Archaic diction is also known as archaism. When a person uses this method of speech, he or she is using words that are old-fashioned and belong in an older era. Poetry is the primary area where archaic words find uses, but plays and books can also contain them. One example is the presence of the word steed in place of a more modern word like horse or stallion.
Language evolves and changes over time. The English that William Shakespeare spoke and wrote in is very different, for example, to the English that Americans and British people speak today. Even in Shakespeare's day, there were words that were out of fashion and belonged to a previous era.
Poetry is an art form that necessitates the use of descriptive and evocative words. Therefore, a poet may prefer to use an archaic word instead of its modern counterpart. The sound of the older word may suit the poem better in terms of rhyme, assonance, or alliteration. It may suit the meter of the poem better than a newer word with a different number of syllables.
The intrinsic fact of a term being archaic, and therefore less common, may give the poem more mystery than a more easily recognizable word. As well as poets, people who write plays and those who write books may use archaism for some of the same reasons. When a writer sets a story in a particular era, he or she may also use archaic diction to make the story more realistic.
Use of old fashioned terminology is not restricted to modern poetry. Certain historical eras of poetry featured the use of archaism. Samuel Coleridge, John Keats, and Alfred, Lord Tennyson all wrote poems in older languages than the usual 19th-century English. Most poets and writers, though, write in the language they speak, and the contemporary audience can understand all the words in the resultant works.
A problem with writing in archaic diction is that many people do not understand the old wordings for familiar concepts. While archaism is always present in old writings and poetry, they typically require translation into modern language for readers to understand. A poet who deliberately adds archaism to his or her poetry could alienate a large part of the audience who do not wish to spend time interpreting the unfamiliar words. Many archaic words, however, such as steed, are commonly understood, although people generally reserve the use of such words for special situations, such as humor.