Hierarchy - Terminology

Terminology

Hierarchies have their own special vocabulary. These terms are easiest to understand when a hierarchy is diagrammed (see below).

The generic hierarchy uses the following terms:

• Object: one entity (e.g., a person, department or concept) or element of arrangement or member of a set
• System: the entire set of objects that are being arranged hierarchically (e.g., an administration)
• Dimension: another word for "system" from on-line analytical processing (e.g. cubes)
• Member: an (element or object) in a (system or dimension) at any (level or rank)
• Rank: the relative value, worth, complexity, power, importance, authority, level etc. of an object
• Level: a set of objects with the same rank OR importance
• Ordering: the arrangement of the (ranks or levels)
• Hierarchy: the arrangement of a particular set of (ranks or levels) i.e. multiple hierarchies are possible per (dimension or system)
• Collection: all of the objects at one level
• Superior: a higher level or an object ranked at a higher level (parent or ancestor)
• Subordinate: a lower level or an object ranked at a lower level (child or descendent)
• Hierarch, the top level of the hierarchy, usually consisting of one object or member of a dimension
• Peer: an object with the same rank (and therefore at the same level)
• Neighbour: the adjacent level/ranking (the immediate superior and immediate inferior)
• Interaction: the relationship between an object and its direct superior or subordinate (i.e. a superior/inferior pair)
  • a direct interaction occurs when one object is on a level exactly one higher or one lower than the other (i.e., on a tree, the two objects have a line between them)
• Distance: the minimum number of connections between two objects, i.e., one less than the number of objects that need to be "crossed" to trace a path from one object to another
• Span: a qualitative description of the width of a level when diagrammed, i.e., the number of subordinates an object has

(N.B., while hierarchies are commonly studied using graph theory, the general terminology used is different, and words such as "direct" may have different general meanings)

Most hierarchies use a more specific vocabulary pertaining to their subject, but the idea behind them is the same. For example, with data structures, objects are known as nodes, superiors are called parents and subordinates are called children. In a business setting, a superior is a supervisor/boss and a peer is a colleague.