Hierarchy
Letter HOrder of importance and containment among concepts: what is primary, what belongs under what, and what appears first.
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Working definition
Hierarchy orders what exists: what is primary, what is contained in what, and what appears first.
It is not depth for its own sake. A deep, symmetrical tree can be harder to traverse than an irregular, recognizable one. In IA, hierarchy and taxonomy overlap, but they are not the same.
Criterion
If someone cannot tell what matters most or what each thing depends on, the problem is hierarchy before layout.