A data dictionary, or metadata repository, as defined in the IBM Dictionary of Computing, is a "centralized repository of information about data such as meaning, relationships to other data, origin, usage, and format".[1] Oracle defines it as a collection of tables with metadata. The term can have one of several closely related meanings pertaining to databases and database management systems (DBMS):

  • A document describing a database or collection of databases
  • An integral component of a DBMS that is required to determine its structure
  • A piece of middleware that extends or supplants the native data dictionary of a DBMS