If we talk about the Application_Domain, It is a mechanism (it's similar to a process in an operating system) to used within the CLI(Common Language Infrastructure) to isolate executed the software_applications from one other so that they do not affect each other. Every Application_Domain has its own virtual address space which scopes the resources for the application domain using that address space.

In the Other word

".Net Application Domain"

This is a virtual process and is used to isolate applications from one another. Every Application_Domain has their own virtual address space which scopes the resources for the application domain using that address space.

All applications running within its main process limitation and its application domain boundaries. Each objects created within the same application scope are created within the same application domain. Various application domains can exist in a single operating system process. The application running inside the single application-domain cannot directly access the code running inside the other application-domain.