Source : Free On-Line Dictionary of Computing

data dictionary
     
         A data structure that stores {meta-data}, i.e. data
        about {data}.  The term "data dictionary" has several uses.
     
        Most generally it is a set of {data descriptions} that
        can be shared by several applications.
     
        Usually it means a {table} in a {database} that stores the
        names, {field} {types}, length, and other characteristics of
        the fields in the database tables.
     
        An active data dictionary is automatically updated as changes
        occur in the database.  A passive data dictionary must be
        manually updated.
     
        In a {DBMS}, this functionality is performed by the {system
        catalog}.  The data dictionary is a more general software
        utility used by designers, users, and administrators for
        {information resource management}.
     
        The data dictionary may maintain information on system
        hardware, software, documentation, users, and other aspects.
     
        Data dictionaries are also used to document the database
        design process itself and can accumulate meta-data
        ready to feed into the system catalog.
     
        [Does anybody call them "codebooks"?]
     
        (2001-04-24)