Microsoft is renaming the app Microsoft Defender.
In Windows 8 and Windows 10, Defender is based on the Microsoft Security Essentials app which offers better protection by adding full blown protection against all kinds of malware. Earlier versions of Windows like Windows 8.1, Windows 8, Windows 7 and Vista also had it but it was less efficient previously as it only scanned spyware and adware.
Windows Defender is the default antivirus app shipped with Windows 10.
