I think of it as "Windows Subsystem for [running] Linux".

The architecture descends from the Windows NT architecture:

   The user mode layer of Windows NT is made up of the "Environment subsystems", which run applications written for many different types of operating systems, and the "Integral subsystem", which operates system-specific functions on behalf of environment subsystems. 

Though as other users pointed out, in WSL 2, the name is inaccurate.