An arrangement of items (objects, names, values, categories, etc.) in which the items are represented as being "above," "below," or "at the same level as" one another and with only one "neighbor" above and below each level. These classifications are made with regard to rank, importance, seniority, power status or authority. A hierarchy of power is called a power structure. Abstractly, a hierarchy is simply an ordered set or an acyclic graph.