A hierarchy is an organizational structure in which items are ranked according to levels of importance. Most governments, corporations and organized religions are hierarchical.
In a computing context, there are various types of hierarchical systems. Most file systems, for example, are based on a hierarchical model in which files are placed somewhere in a hierarchical tree model. A file is placed in a directory (folder in Windows) or subdirectory at the desired place in the tree structure. The computer memory hierarchy ranks components in terms of response times, with processor registers at the top of the pyramid structure and tape backup at the bottom.
The word hierarchy comes from the Greek hieros, meaning sacred, and archein, meaning to rule. The word first entered English in the fourteenth century. According to the Oxford English Dictionary, it originally referred to “each of the three divisions of angels, every one comprising three orders, in the system of Dionysius the Areopagite.”