A set of data that the Weave server can use in various ways, for example to provide a list of values for the user when entering data, or as a table to display textual data about a selected entity, anywhere that Weave needs some textual data it will retrieve it via a data definitionData Definition.
The Data Definition itself is just a container for a data connection, the Data Definition provides an id and a container for a data connection and it 's is the data connection that describes the source of the underlying data and how that data is obtained.