Pronounced like the word dairy this fictional small city of about 100,000 is featured in several books by Stephen King. The first is the massive novel IT and the second is Insomnia. It is also mentioned in Bag of Bones, The Tommyknockers, and Storm of the Century. Derry is located close to Bangor and Haven Maine.
It is often used as an example of people living with evil. The residents of the town tend to not see when bad things happen and not be disturbed by things like an abnormally high murder rate. Indeed they tend to benefit from the presence of IT. The city gets a significant bit of economic prosperity in spite of the problems the town has. The residents also tend to be bigoted in some form.
This comes to an end when IT is finally destroyed. Then the town begins what will probably be a long painful decline without ITs supernatural support.