TIL they are called "hives". Windows Registry is an interesting thing. Even casual users have to interactive with it once or twice w/o fully understand it.
Raymond Chen explained why a registry file is called a “hive”:
Because one of the original developers of Windows NT hated bees. So the developer who was responsible for the registry snuck in as many bee references as he could. A registry file is called a “hive”, and registry data are stored in “cells”, which is what honeycombs are made of.