Definition - What does Windows Defender mean?
Windows Defender is a software application that safeguards a system from malware. It was an anti-spyware program built to fight unauthorized access and protect Windows computers from unwanted software. Introduced with the Windows Vista installation pack, it is now available for free download as part of Microsoft Security Essentials.
Windows Defender was previously known as Microsoft AntiSpyware.
Techopedia explains Windows Defender
Released in 2006, Windows Defender was a built-in anti-spyware application included with Windows Vista and Windows 7, and was later updated to include support for Windows XP and Windows Server 2003. Windows Defender was later incorporated into Microsoft Security Essentials, which targets a wider range of malware, and is available as a free download.
Windows Defender was updated with the release of Windows 8. In Windows 8, rather than focusing solely on spyware, Windows Defender offers virus protection as well, similar to Microsoft Security Essentials.