IfcPropertyDefinition defines the generalization of all characteristics (i.e. a grouping of individual properties), that can be assigned to objects. Currently, subtypes of IfcPropertyDefinition include property set occurrences, property set templates and property templates. 

• Property set template - IfcPropertySetTemplate, a collection of property templates that determine the definition of properties used within a project context.
• Property template - IfcPropertyTemplate, a single template that determines the definition of a particular property used in the same project context. The template may determine the name, description, data type, the unit, or a standard expression for each property that is based on that template.
• Property set occurrence - IfcPropertySet, a set of individual properties (that may or may not be determined by a property template) holding individual values, measure types and units, and are associated to an object occurrence or object type.

Property definitions define information that is shared among multiple instances of objects, either object occurrences or object types. IfcPropertyDefinition's (by their instantiable subtypes) can participate within the following relationships:

• Assignment to a project context - an HasContext relationship to IfcRelDeclares that establishes the project context in which this property definition is declared. This relationship is mostly applicable to subtypes of IfcPropertyTemplateDefinition.
• Association to external resources - an HasAssociation relationship to IfcRelAssociates that refers to external sources of information (most notably a classification or document) and creates a uni-directional association. There is no dependency implied by the association.

• The subtype hierarchy of IfcPropertyDefinition also includes statically defined property sets as IfcPreDefinedPropertySet. Those are rarely used collections of fixed attributes combined in an entity definition. The IfcPreDefinedPropertySet can not be determined by an IfcPropertySetTemplate.
• Individual properties, (subtypes of IfcProperty), are currently not included in the subtype hierarchy of IfcPropertyDefinition. This is due to upward compatibility reasons with earlier releases of this standard.