Wikipedia
Interix was an optional, POSIX-conformant Unix subsystem for Windows NT operating systems. Interix was a component of Windows Services for UNIX, and a superset of the Microsoft POSIX subsystem. Like the POSIX subsystem, Interix was an environment subsystem for the NT kernel. It included numerous open source utility software programs and libraries. Interix was originally developed and sold as OpenNT until purchased by Microsoft in 1999.
Interix versions 5.2 and 6.0 were respective components of Microsoft Windows Server 2003 R2, Windows Vista Enterprise, Windows Vista Ultimate, and Windows Server 2008 as Subsystem for Unix-based Applications (SUA). Version 6.1 was included in Windows 7 (Enterprise and Ultimate editions) but disabled by default, and in Windows Server 2008 R2 (all editions). It was available as a deprecated separate download for Windows 8 and Windows Server 2012, and not available at all on Windows 10.