How do you represent an application in UML?

Is it as simple as just a square or开发者_JS百科 rectangle with the application name in the center?

If by application, you mean an other (external) system which interact with your system (application) then it is an actor. You can stereotype that actor as application and in some software gives it a specialized icon (square or whatever). Unfortunately all UML softwares are very poor in representation so sometimes it's better to use a graphics software to draw nice UML diagrams.

I have been using UML to describer enterprise level architetures, where in a component diagram one component per application and in sequence diagrams one lifeline per application was used.

The square or ractangle define the system not the application in UML. Also at the when you prepare UML, you dont have an application. You only have a problem that belong to a system that have some actors and some process ( use cases ).

So there is no point to include application in UML.