For issues relating to arranging items into ordered levels.
A hierarchy is an arrangement of items (objects, names, values, categories, resources, etc.) in which the items are conceptually "above," "below," or "at the same level as" one another. Abstractly, a hierarchy is simply an ordered set or an directed acyclic graph.
The items being organized are normally refereed to as nodes, entities, or elements. A hierarchy can link entities directly or indirectly, and either vertically or horizontally. The only direct links necessary to establish hierarchy are to node's immediate superior (ie. "parent"), or to the node's subordinates (ie. "children").
However, more complex relationships can be inferred through indirect linkage, or linkage by proxy of other nodes. Conceptually, you don't have to link to a node's grandparent, it can be inferred that the parent's parent is a grandparent. Conversely, a sibling -- related horizontally to the node -- can be said to be any child of your parent and likewise no "direct" linkage is required.