Definition of .NET Application Domain:

An application domain is a logical isolation border placed around .NET applications to prevent them from interacting with or affecting one another. It’s a simple process with its own code, data, and settings. Runtime hosts, which are called by the common language runtime (CLR) to load the programmes to be executed, produce application domains.

The processes in which programmes were loaded served as the isolation border before .NET. Every process has its own virtual memory and cannot directly access the memory of another process, and a process’s features are similar to those of the application domain.

For security, reliability, versioning, and unloading assemblies, application domains provide a separation boundary. Runtime hosts are in charge of bootstrapping the common language runtime before an application is executed; hence they often build application domains.

.NET Application Domain Benefits:

• Process boundaries have traditionally been used to separate apps that share the same computer. Each application is loaded into its own process, separating it from other programmes on the same computer. 
• Because memory addresses are process-relative, the applications are isolated. A memory pointer passed from one process to another cannot be used in any meaningful way by the target process. Furthermore, direct calls between processes are not possible. Instead, you’ll have to rely on proxies, which provide some degree of indirection. 
• Before managed code can be used, it must first go and pass a verification process (unless the administrator has granted permission to skip the verification). The verification procedure examines if the code may access invalid memory addresses or take other actions that could cause the process in which it is running to fail. Passing code It’s claimed that the verification test is type-safe. Because type-safe code can be verified, the common language runtime can provide the same amount of isolation as the process border at a significantly lower performance cost.
• Increases server scalability considerably.