An object property is one of two main categories of properties. It links individuals to individuals, and is defined as an instance of the built-in OWL class owl:ObjectProperty. This defines a property with the restriction that its values should be individuals.
An owl:ObjectProperty is a subclass of the RDF class
A property axiom defines characteristics of a property. In its simplest form, a property axiom just defines the existence of a property. For example:
The second example below states that all instances (pairs) contained in the property extension of the property "hasMother" are also members of the property extension of the property "hasParent":
In OWL Full, object properties and datatype properties are
not disjoint. Because data values can be treated as individuals, datatype
properties are effectively subclasses of object properties. In OWL Full
owl:ObjectProperty is equivalent to
practice, this mainly has consequences for the use of
We use the term "property extension" in a similar fashion to "class extension". The property extension is the set of instances that is associated with the property. Instances of properties are not single elements, but subject-object pairs of property statements. In relational database terms, property instances would be called "tuples" of a binary relation (the property).