A hierarchy (from Greek: ἱεραρχία, hierarkhia , 'rule of a high priest',
from hierarkhes , 'president of sacred rites') is an arrangement of items
(objects, names, values, categories, etc.) that are represented as being
"above", "below", or "at the same level as" one another. Hierarchy is an
important concept in a wide variety of fields, such as architecture,
philosophy, design, mathematics, computer science, organizational theory,
systems theory, systematic biology, and the social sciences (especially
political science).
A hierarchy can link entities either directly or indirectly, and either
vertically or diagonally.