[masklinn]

It's a Windows subsystem[0] for [running] Linux [applications].

The terminology is "Windows subsystem for [target environment]" so "Windows subsystem for OS/2" lets you run OS/2 application on NT, "Windows subsystem for Win32" lets you run Win32 applications on NT, and "Windows subsystem for Linux" lets you run Linux applications on NT.

[0] "The interface between user mode applications and operating system kernel functions […] There are four main environment subsystems: the Win32 subsystem, an OS/2 subsystem, the Windows Subsystem for Linux and a POSIX subsystem."

[joombaga]

Yet they chose different terminology for WoW64 (Windows 32-bit on Windows 64-bit), and chose to store 32-bit binaries in the WoW64 directory.

[masklinn]

I don't know that Win64/WoW64 is a separate subsystem. I think it's a subset of the 64b version of the Win32 subsystem.

[signal11]

The Architecture of Windows NT page on Wikipedia has a good diagram to help visualize NT subsystems. The first few versions of NT, iirc, shipped with the OS/2 subsystem but this was removed with Windows XP.

https://en.wikipedia.org/wiki/Architecture_of_Windows_NT