To effectively isolate a directive's $scope, the __________ property is used in the directive's definition.

To effectively isolate a directive's $scope, the scope property is used in the directive's definition. By setting the scope property to true or an object, you can create an isolated scope for the directive. This helps in encapsulating the directive's behavior and preventing unintended interactions with parent scopes. Understanding scope isolation is essential for building modular and reusable directives in AngularJS applications.