The Free On-line Dictionary of Computing (30 December 2018):

twm

   Tab Window Manager.

   A window manager for the X Window System.  Twm provides
   titlebars, shaped windows, several forms of icon management,
   user-defined macro functions, click-to-type and
   pointer-driven keyboard focus, and user-specified key and
   pointer button bindings.  It can be extensively configured by
   a startup file.

   Twm was written by Tom LaStrange, Solbourne Computer; Jim
   Fulton, MIT X Consortium; Steve Pitschke, Stardent
   Computer; Keith Packard, MIT X Consortium; Dave Sternlicht,
   MIT X Consortium; Dave Payne, Apple Computer.

   An extended version, vtwm, provides a virtual desktop.

   [Why "Tab"?]

   (1995-02-14)