What does UML "redefines" mean?

The UML 2.5.1 defines redefinition in section 9.2.3.3 (page 100):

Any member (that is a kind of RedefinableElement) of a generalization of a specializing Classifier may be redefined instead of being inherited. Redefinition is done in order to augment, constrain, or override the redefined member(s) in the context of instances of the specializing Classifier.

For a feature such as an attribute, a property, or an operation:

Feature redefinitions may either be explicitly notated with the use of a {redefines <x>} property string on the Feature or implicitly by having a Feature which cannot be distinguished using isDistinguishableFrom() from another Feature in one of the owning Classifier’s more general Classifiers.

Suppose for example that you have a class Node with two attributes: from: Node[*] and to[*]: Node. You could then have a specialization FamilyMember (a node in your genealogy) and you could redefine the members: parent : FamilyMember[*] {redefines from} and child : FamilyMember[*] {redefines from}

Another example: you have a polymorphic class Shape with an abstract operation draw(). You can specialize that class into a class Circle that will have its own draw(). You could leave the redefinition implicit (just mentioning the operation), or you could be very explicit with draw() {redefines draw()}.

The abstract syntax diagrams apply UML to the UML metamodel. The redefinition have the same meaning, but it is explained in a shorter manner in section 6.

Let's take an example in your diagram: let's take IntervalConstraint:

• IntervalConstraint inherits from Contraint. A Constraint is composed of a property specification:ValueConstraint (page 36), so IntervalConstraint inherits this property.
• Your diagram tells that IntervalConstraint is composed of a property specialization: Interval that redefines the more general specification of the constraint. It's a redefinition, because it narrows down its type (fortunately, Interval inherits from ValueSpecification so there's no risk of inconsistency).