Summary form only given. UML (Unified Modelling Language) is a universal language for modelling of object-oriented applications. UML can build models of different kinds of domains. However, no paradigm is enough to clearly model all views of all possible domains. For this reason, there are several metamodel elements to extend UML and facilitate the modelling of specific domains. We analyze three kinds of existing extensions to the UML metamodel and propose another extension, evolutionary stereotype. The abstract syntax, well-formedness rules and semantics are defined. A case study is written to show how the dynamic semantics is specified. These stereotypes allow integrating not only the possibility to specify restrictions to the metamodel, in particular using OCL (Object Constraint Language), but also to aggregate new semantic definitions. The UML metamodel is organized in logic packages. One of them is the extension mechanisms package where the extensions to UML are specified. The evolutionary stereotypes are defined as a new metaclass of the extension mechanisms package. This new metaclass has associations that relate this metaclass with existent metaclasses of the OMG (Object Management Group) metamodel.