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

light-weight process

    (LWP) A single-threaded
   sub-process which, unlike a thread, has its own process
   identifier and may also differ in its inheritance and
   controlling features.

   Several operating systems, e.g. SunOS 5.x, provide system
   calls for creating and controlling LWPs.

   (1996-02-22)