What is a Data Dictionary?

The Data Dictionary is a CSV file that holds the instruments of a project. In addition to building an
instrument using Online Designer, instruments can be built or edited in a Data Dictionary, then uploaded
into REDCap. Once a user is familiar with building projects in Online Designer and comfortable with Data
Dictionary syntax, this can be an efficient way to build larger projects or make large scale changes. The
Data Dictionary will not contain any information about survey settings, events in a longitudinal study, or
other project-level settings.

The University of Illinois has an excellent guide to using Data Dictionaries available here