Definition queries are used to create a new data source out of a subset of an existing data source. Only Site Admins have permission to create Definition Queries.

Here is how to create a Definition Query:

1. Open the LAYER ADMIN panel and click on the Data Sources tab.
   
2. Click the "NEW DATA SOURCE" button at the bottom of the screen.
   
3. Select the data source that you wish to create a new data source from. Click SELECT.
   
   
4. Name your new data source.
   
   
5. Click inside the 'Definition Query' field to build your filter.
   
   
   
6. Build a filter to tell the site what subset of the original data source you wish to create.
   
   
7. Click OK, then click CREATE. 
   
   

This will create both a layer and a data source for the new subset of the existing data source. The site will automatically open the layer in edit mode so that you can go straight into editing the layer's style, label, etc!