While colloquially the two terms are often used to refer to the same thing, they are not, in fact, the same thing.
Windows Defender is the desktop security client default for Microsoft Windows OS and other OSes.
Microsoft Defender is a broader suite of security tools that includes Windows Defender for Endpoint (a variant of Windows Defender) that is used for enterprise security particularly for Microsoft 365 licensees. It includes both endpoint clients and server and infrastructure security tools. You'll see the Microsoft Defender branding applied to the security.microsoft.com portal when you have a Microsoft 365 license.
Note that this distinction has not always been respected by Microsoft, and you will see their own answers and documentation referring to the desktop client, in particular, as both Microsoft- and Windows-Defender in the past. It is more recently with their cloud efforts and development of the enterprise-class security products that the two terms have taken on distinct meanings.
Suggested solution: The Windows Defender tag should be used. Microsoft Defender, being typically an enterprise utility, should include a recommendation to either tag correctly, if the person is asking about their own PCs security software, or to ask on ServerFault or Security, if their question is regarding the enterprise product and security.
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