A POSIX-based Unix environment from Microsoft that runs on Windows clients and servers. Subsystem for UNIX-based Applications (SUA) provides a native Unix environment that uses the Windows kernel, but also includes a software development kit (SDK) that allows Unix programs to be recompiled for Windows.
A Lot of Subsystem Branding
In 1996, Microsoft introduced OpenNT, its first Unix subsystem for Windows NT. In 1998, it was rebranded as Interix and remained as a separate product until it became part of Microsoft Windows Services for UNIX (SFU) in 2002.
SFU was rebranded as Windows Server 2003 Subsystem for UNIX-based Applications and Windows Vista Subsystem for UNIX-based Applications in 2005 and 2006, respectively. See POSIX