ASP.NET introduces a concept of application domain or AppDomain which is like a lightweight process that acts like both container and boundary. The .NET run-time uses the AppDomain as a container for data and code. The CLR allows multiple .NET applications to run in a single AppDomain.

In .NET, an application domain, often abbreviated as AppDomain, is a lightweight process that serves as an isolated environment in which applications execute. It provides a boundary within which code runs, ensuring that any errors or exceptions that occur in one application domain do not affect other application domains.

Here’s a more detailed explanation:

Isolation: Application domains provide a level of isolation, allowing multiple applications to run within the same process without interfering with each other. This isolation helps in enhancing security, stability, and manageability of applications.
Security: Since each application domain is isolated, it provides a level of security by preventing code in one domain from accessing or affecting code in another domain without appropriate permission.
Memory Management: Application domains have their own memory space, allowing for more efficient memory management. When an application domain is unloaded, all resources associated with it, including memory, are released.
Versioning: Application domains help in managing different versions of assemblies within the same process. This allows applications to use different versions of the same assembly simultaneously without conflicts.
Exception Handling: Exceptions that occur within an application domain are confined to that domain, preventing them from affecting other parts of the application.

In summary, an application domain provides a controlled and isolated environment for executing .NET applications, contributing to security, stability, and efficient resource management.