When you have a posh and formal way of speaking, this is an example of proper diction.
A choice to use poetic language in writing is an example of diction.
Origin of diction
- Middle English diccion a saying, word from Old French from Latin dictiō dictiōn- rhetorical delivery from dictus past participle of dīcere to say, speak deik- in Indo-European roots
From American Heritage Dictionary of the English Language, 5th Edition