The word amity refers to a peaceful, friendly nature, much like the French word for friend — "ami."
From the Latin amicus "friend," amity means "friendly relations." That strong sense of friendship lends irony to the name of the book, and later the movie, called "The Amityville Horror," about a horrifying town with a weirdly cheerful name. In truth, though, amity means mutual understanding and peace. Wouldn't it be nice if all of the world's nations lived in amity?