What is stereotype in UML?

Stereotype is a profile class which defines how an existing UML metaclass may be extended as part of a UML profile. Profiles allow lightweight extension of the standard UML by defining custom stereotypes, tagged values, and constraints for different:

• platforms (such as J2EE or .NET), or
• domains (such as real-time or business process modeling).

In other Words, if existing UML metaclasses are not good enough, you can add your custom stereotypes by modifying standard UML metaclasses.