"An arrangement of members of a dimension into levels based on parent-child relationships, such as Year, Quarter, Month, and Day or Country, Region, State or Province, and City. Members in a hierarchy are arranged from more general to more specific." (Microsoft Corporation, "SQL Server 7.0 System Administration Training Kit", 1999)
"A navigation path that allows the user to move from summarized to detailed information with each level of the hierarchy represented by a different attribute." (Reed Jacobsen & Stacia Misner, "Microsoft SQL Server 2005 Analysis Services Step by Step", 2006)
"A series of master-detail relationships within a dimension. Many front-end tools require hierarchy definitions in order to support drilling features. Some aggregate navigation and construction tools, such as Oracle's materialized views, require explicit declaration of a hierarchy if a dimension is to be partially summarized by an aggregate fact table." (Christopher Adamson, "Mastering Data Warehouse Aggregates", 2006)
"A logical tree structure that organizes the members of a dimension such that each member has one parent member and zero or more child members." (Microsoft, "SQL Server 2012 Glossary", 2012)
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