Usually when writing poems or literature, the authors looks for words that can convey their ideas faithfully. The same thing happens to the readers, when suddenly they understand the problem so clear. Mostly they're already available in the language, but sometimes one need to create a new one. In my language that kind of words are called "fine words", but how would English call it? It's the same thing with technical terms in academic fields, but what is an equivalent term in poetry?
One possible term is mot juste, which means (in English),
the exactly right word or phrasing. See Merrian-Webster.
I believe it also means the same thing in French. We generally use the French phrase in English because there's no good equivalent English word.
If you're referring to the creation of a completely new word, "neologism" (a new word, usage, or expression) might be what you're looking for. However, neologism doesn't have a value judgement associated with it: a neologism could be good or bad.