I did an experiment in my instructor-led classes. For five classes, I did not have the students work on a data dictionary before they began object modeling. For five more classes, I required them to draft a data dictionary before they began object modeling. They all did the exact same workshop.
The first five classes, without the data dictionary, took an average of 6 to 8 hours to complete the workshop object model.
The second five classes, which already had a data dictionary in hand, took only 3 to 4 hours.

A data dictionary is a "centralized repository of information about data such as meaning, relationships to other data, origin, usage, and format." The term may have one of several closely related meanings pertaining to databases and database management systems (DBMS):

1. a document describing a database or collection of databases
2. an integral component of a DBMS that is required to determine its structure
3. a piece of middleware that extends or supplants the native data dictionary of a DBMS.