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. 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. This name change can be seen in the Windows Security settings as shown below. There are still other components that have not been renamed. For example, this build of Windows 10 continues to use the Windows Defender name for Windows Defender Firewall with Advanced Security, Windows Defender Application Guard, and Windows Defender SmartScreen.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-defender-gets-a-new-name-microsoft-defender/