Win32s

Win32s is a Microsoft upgrade to the Windows 3.1 and Windows for Workgroups 3.1 operating systems, which run 16-bit applications, that allows them to run some 32-bit applications. Users of Windows 3.1 or Windows for Workgroups 3.1 who did not want to install Windows 95 (which was designed for 32-bit applications) could download Win32s and install it so that certain 32-bit applications would run.

The NCSA Mosaic browser was an example of a 32-bit application that would run on Windows 3.1 if you downloaded Win32s.