What is the term for special words that don't use standard readings or regular modifications?


Words like 大人、下手、上手... as おとな, へた, じょうず.  
What is the name for terms of this type?

Not including: [Kanji that don't use their specified
readings](https://japanese.stackexchange.com/questions/38728/kanji-that-dont-
use-their-specified-readings).  
The answers for おみやげ don't answer the classification of non-standard
pronunciation (they answer other questions but the stated one).  
The おとな related question is very specific to that word.  
I believe the other stated duplicate doesn't answer this in general either. It
would very hard for a searcher to find these somewhat duplicate questions.

