ordered ranking of levels where each level contains or governs the one below
biological taxonomy: domain, kingdom, phylum, class, order, family, genus, species
organizational: executive, management, operational layers
protocol layers: OSI model (physical, data link, network, transport, session, presentation, application)
file systems: root, directories, subdirectories, files
Maslow's hierarchy of needs: physiological, safety, belonging, esteem, self-actualization
Chomsky hierarchy: regular, context-free, context-sensitive, recursively enumerable languages
holarchy: each level is both a whole and a part of a larger whole (Arthur Koestler's holon)
hierarchies enable modularity, abstraction, and scalable organization