What is the difference between process and AppDomain in .NET?

A Process is an OS-level isolation boundary — its own memory space, handles, and security context. Starting a separate process provides complete isolation. A AppDomain (Application Domain) was a .NET Framework feature providing a lightweight isolation boundary within a single process — allowing multiple independent "mini-applications" to run in one process with isolated state and different permissions, enabling unloading of code. Uses: hosting environments (IIS, older plugin systems), isolated test runs. In .NET Core / .NET 5+: AppDomains are removed (except for the default AppDomain which exists for compatibility). Code that creates new AppDomains throws PlatformNotSupportedException. The replacement for runtime code isolation in .NET Core is: (1) AssemblyLoadContext (ALC): load and unload assemblies into isolated contexts — the new plugin system. (2) Separate processes. (3) Isolated containers/processes. AssemblyLoadContext.Collectible = true enables unloading of the loaded assemblies.