Originally, Windows Defender
was a simple anti-spyware program offered by Microsoft. It was not a full anti-malware suite. I also think it was offered as part of the installed programs with Windows 7. In 2009 (if I remember correctly) Microsoft offered a full anti-malware suite and called it Microsoft Security Essentials and pretty much phased out Windows Defender.
Fast forward and Microsoft offered yet another product called Windows Defender Offline
which is a free standalone, bootable malware and virus remover that performs an offline scan of an infected PC to remove viruses, rootkits and other advanced malware.
Then when Windows 8 was released, Microsoft once again decided to confuse people by calling the provided anti-malware suite Windows Defender
even though, for all intents and purposes, it is identical to Microsoft Security Essentials.