Hierarchy
Etymology
From Greek "hierarkhia," combining "hieros" (sacred) and "arkhein" (to rule) — originally the ranked orders of angels and church officials. It literally meant rule of the holy, a divinely ordered ladder. The sacred faded and the structure remained, generalizing to any layered order.
noun · /HY-uh-rar-kee/
A system in which people or things are ranked in levels of authority or importance.
What it names
The invisible ladder inside any group, so you can see who reports to whom and how decisions travel.
Example
The flat startup grew until an unspoken hierarchy formed and nobody knew who could actually approve things.
Related idea
A hierarchy is the chain of authority; a bureaucracy is the machinery of rules that a large hierarchy tends to grow. You can flatten a hierarchy and still drown in bureaucracy, and vice versa.