In MicrosoftDotNet, a unit of execution smaller than a process, but with many of the same characteristics - isolation of faults, being independently stoppable or debuggable, isolation of data where not explicitly shared, unit of security policy, and so forth.

By default, a MicrosoftDotNet application runs in a single ApplicationDomain - subsidiary ApplicationDomains might be used, for example, to run applets in a WebBrowser, MacroLanguage code in any application, or other such code where it might be desirable to limit its capabilities to a subset of those of its parent ApplicationDomain.

http://msdn.microsoft.com/library/en-us/cpguidnf/html/cpconapplicationdomains.asp
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CategoryDotNet