Well, that's the official designation, I guess. Microsoft Defender. Doesn't have any ooomph to it.
"Windows Security" is still shown on the Defender main page "Security at a Glance." And "Windows Defender Firewall" is in the Control Panel. Silly, right? Far as I'm concerned, it's simply "Defender."
For the record, MS has decided to discontinue "DisableAntiSpyware", so the only proper way to disable WD will be by installing 3rd party AV.
This setting is discontinued and will be ignored on client devices, as of the August 2020 (version 4.18.2007.8) update to Microsoft Defender Antivirus.