Grow up.

"Diction" applies to both the spoken and written word. Here is Britannica's more lengthy description and the example fragment is from a writer:

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Choice of words, especially with regard to correctness, clearness, or effectiveness. Any of the four generally accepted levels of diction—formal, informal, colloquial, or slang—may be correct in a particular context but incorrect in another or when mixed unintentionally. Most ideas have a number of alternate words that the writer can select to suit his purposes. “Children, …
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"Diction." Encyclopædia Britannica. 2004.  Encyclopædia Britannica Premium Service.
http://www.britannica.com/eb/article?eu=30843

-Marty