A model is a group of elements, such as metadata and source files that typically constitutes the solution. A model is a design-time concept and typically represents a solution area, for example, the general ledger model and project model. A model always belongs to a package.  

A model concept was also available in Dynamics AX 2012. In Dynamics AX 2012, you can create one or more models in a layer. All the models are stored in a database.