A solution to the problem when Windows Update fails at updating Windows Defender in Windows 8.1 and especially when the problem is related to corrupted definitions is to remove all definitions using the following steps:
1. Run “Command prompt” as administrator
2. cd “C:\Program Files\Windows Defender”
3. MpCmdRun.exe -RemoveDefinitions -All
4. Update to the latest definitions through “Windows Defender” in “Control Panel”
The next Windows Update should work as normal.