Chapter 11: Diction & Pronunciation
Your diction is simply your choice of words. Diction is closely linked with the author’s tone or register.
"... [A] register is a variety of a language used for a particular purpose or in a particular social setting. For example, when speaking in a formal setting contrary to an informal setting, an English speaker may be more likely to use features of prescribed grammar—such as pronouncing words ending in -ing with a velar nasal instead of an alveolar nasal (e.g. "walking", not "walkin'"), choosing more formal words (e.g. father vs. dad, child vs. kid, etc.), and refraining from using words considered nonstandard, such as ain't. " - Wikipedia
There is no single, correct diction in the English language; instead, you choose different words or phrases for different contexts:
All of these expressions mean the same thing -- that is, they have the same denotation -- but you would not likely switch one for the other in any of these three situations: a police officer or employer would take "screw-up" as an insult, while your friends at the bar after a hockey game would take "oversight" as an affectation.
When writing or speaking in an English course, it is expected that students use the "formal" register. In other words, students should express themselves as though they were in an academic or professional situation.