How to Fix Windows Defender When It Won't Update Virus Definitions

I have been fighting to get Windows Defender updated for a couple days and stumbled on this fix.
I kept getting error 80070643 whenever I tried to update the definitions, either through Windows Update, or from inside Defender.

Here's what worked for me
Open CMD as Admin and typing (with quotes):

"%PROGRAMFILES%\Windows Defender\MPCMDRUN.exe" -RemoveDefinitions -All

"%PROGRAMFILES%\Windows Defender\MPCMDRUN.exe" -SignatureUpdate