When writing Fantasy, Historical Novels, or even Science Fiction - language patterns and terms are incredibly different back then (or in the future), then we have now.
How can you properly convey an idea that doesn't exist, for example, the word psychology, in such a setting? For example, a character understands the general gist of it - and you can explain it, but how can you summarize it without having to explain it every time? Should you invent a new word for it - or use the modern rendition of the word?
The same applies to things such as inventions; assume a plot is set in the dark ages, and an invention for communication through the use of 'magic' is created. Should it be referenced to using a new word, or would it be better to use common words such as 'calling' and 'reception' that the reader would understand better, but may not make sense in the world itself?
One reason I have this conundrum is also because our language has an incredibly rich history, with words such as Psychology having links to greek with historical impacts that influenced its reasoning. It seems wrong to have a character suddenly come up with a word that is in our society, but seems to have appeared from no where in theirs.