Windows Defender is being rebranded to Microsoft Defender to indicate that it now part of a cross platform family of products. This includes enterprise products, which offer solutions for multiple OS platforms.
Historically, Microsoft’s consumer antivirus product has been called Windows Defender. With the release of their enterprise security offerings called Microsoft Defender Advanced Threat Protection (ATP) and Microsoft Defender ATP for Macs, Microsoft has decided to rebrand the Windows version under this family name as well.
In a statement to Bleeping Computer, Microsoft has stated that they are making these changes to show that their security products have become cross-platform under the umbrella name Microsoft Defender.