A definition is an object with no impact on a simulation unless it is assigned to a part, excitation, or sensor. Once that association is made, it determines the electromagnetic behavior of the parent object.
A material definition provides the electromagnetic properties for a part. Once added to a project, a material definition is assigned to a part by either clicking and dragging it, or through the part's right-click menu.
A circuit component definition provides the lumped definition for a circuit component excitation. The excitation specifies the location in space, while the definition determines its behavior.
A sensor definition is associated with a near-field sensor. The sensor specifies the location in space where the data is saved, and the definition determines which data to save.