Poetic diction is the language a poet chooses for a poem: the words, phrases, and style of expression. It can be plain or elevated, modern or archaic, formal or conversational, depending on the effect the poet wants. The term often comes up in discussions of how poetry's language differs from everyday speech, though many modern poems deliberately blur that distinction.
Example
Moonlight silvered the wet stone,
and the old gate whispered in the wind;
I walked home with my sleeves dark with rain.