Working with a Definition 
A definition in the rules composer represents a business term or a local computational variable required in a business rules model.
Example
When processing a list of orderitems, you create a definition called CurrentOrderItem to identify the current item being processed. This section describes the tasks that you perform with definitions.
End of the example.
You can create definitions specifically to be used within a ruleset and flow ruleset. You can also create definitions to be used across the DC.