Good diction means the exact or near exact.

好的表达意味着精确或接近精确。

youdao

His diction wasn't very good.

他的发音不是很好。

《柯林斯英汉双解大词典》

But here I want to point out: not the books in domestic, though they are good, the diction of words still needed improving.

但是在这里我想指出：不在国内书，虽然是好的词中的措辞用语仍需改进。

youdao

Exhibiting style, using passable diction, earning a good salary as a single woman, sleeping with a white man: to Madea, these are all pollutants to the authentic black woman.

NEWYORKER: Mama��s Gun