Definition from IAI: A definition relationship (IfcRelDefines) that uses a type definition or property set definition (seens as partial type information) to define the properties of the object instance. It is a specific - occurrence relationship with implied dependencies (as the occurrence properties depend on the specific properties).

The IfcRelDefines relationship establishes the link between one type (specific) information and several objects (occurrences). Those occurrences then share the same type (or partial type) information.

EXAMPLE: Several instances of windows within the IFC project model may be of the same (catalogue or manufacturer) type. Thereby they share the same properties. This relationship is established by a subtype of the IfcRelDefines relationship assigning an IfcProductType (or subtype thereof) to the IfcWindow.
HISTORY New entity in IFC Release 2x.
ISSUE See issue and change log for changes made in IFC Release 2x.

EXPRESS specification:

ENTITY IfcRelDefines
ABSTRACT SUPERTYPE OF (ONEOF(IfcRelDefinesByProperties, IfcRelDefinesByType))
SUBTYPE OF ( IfcRelationship);
RelatedObjects  :  SET [1:?] OF IfcObject;

Attribute definitions:

RelatedObjects  :  Reference to the objects (or single object) to which the property definition applies.

References (4):

Name Type Referred through Express-G
IfcObject Entity
Attribute 'IsDefinedBy'
Diagram 1
IfcRelDefinesByProperties Entity
Diagram 3
IfcRelDefinesByType Entity
Diagram 3
IfcRelationship Entity
Diagram 1

Inheritance graph

ENTITY IfcRelDefines;
GlobalId  :  IfcGloballyUniqueId;
OwnerHistory  :  IfcOwnerHistory;
Name  :  OPTIONAL IfcLabel;
Description  :  OPTIONAL IfcText;
ENTITY IfcRelationship;
ENTITY IfcRelDefines;
RelatedObjects  :  SET [1:?] OF IfcObject;