A series of ordered groupings of people or things within a system WordNet 3.0 & Open English WordNet

A hierarchy is an arrangement of items Wikipedia

Arrangement of units into related levels of different weights or ranks, meaning that levels are considered "higher" or "lower" than one another. The term, which originally meant rule by priests, is now generalised and describes systems with a linear concept of and and where each level has only 1 direct parent level. Hierarchies are typically depicted as a tree structures. Wikipedia Disambiguation

System of elements with multiple levels or tiers that are subordinated to each other Wikidata

Any group of objects ranked so that every one but the topmost is subordinate to a specified one above it. OmegaWiki