V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

OS2
       Operating System /2 (IBM, OS), "OS/2"


The Jargon File (version 4.4.7, 29 Dec 2003):

OS/2
 /O S too/, n.

    The anointed successor to MS-DOS for Intel 286- and 386-based micros; proof
    that IBM/Microsoft couldn't get it right the second time, either. Often
    called ?Half-an-OS?. Mentioning it is usually good for a cheap laugh among
    hackers ? the design was so baroque, and the implementation of 1.x so
    bad, that three years after introduction you could still count the major 
    apps shipping for it on the fingers of two hands ? in unary. The 2.x
    versions were said to have improved somewhat, and informed hackers rated
    them superior to Microsoft Windows (an endorsement which, however, could
    easily be construed as damning with faint praise). In the mid-1990s IBM put
    OS/2 on life support, refraining from killing it outright purely for
    internal political reasons; by 1999 the success of Linux had effectively
    ended any possibility of a renaissance. See monstrosity, cretinous, 
    second-system effect.


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

OS/2
Merlin
Warp

   /O S too/ IBM and Microsoft's successor to the MS-DOS
   operating system for Intel 80286 and Intel 80386-based
   microprocessors.  It is proof that they couldn't get it
   right the second time either.  Often called "Half-an-OS".  The
   design was so baroque, and the implementation of 1.x so bad,
   that 3 years after introduction you could still count the
   major application programs shipping for it on the fingers of
   two hands, in unary.  Later versions improved somewhat, and
   informed hackers now rate them superior to Microsoft
   Windows, which isn't saying much.  See second-system
   effect.

   On an Intel 80386 or better, OS/2 can multitask between
   existing MS-DOS applications.  OS/2 is strong on
   connectivity and the provision of robust virtual machines.
   It can support Microsoft Windows programs in addition to its
   own native applications.  It also supports the Presentation
   Manager graphical user interface.

   OS/2 supports hybrid multiprocessing (HMP), which provides
   some elements of symmetric multiprocessing (SMP), using
   add-on IBM software called MP/2.  OS/2 SMP was planned for
   release in late 1993.

   After OS/2 1.x the IBM and Microsoft partnership split.
   IBM continued to develop OS/2 2.0, while Microsoft developed
   what was originally intended to be OS/2 3.0 into Windows NT.
   In October 1994, IBM released version OS/2 3.0 (known as
   "Warp") but it is only distantly related to Windows NT.
   This version raised the limit on RAM from 16MB to 1GB (like
   Windows NT).

   IBM introduced networking with "OS/2 Warp Connect", the first
   multi-user version.  OS/2 Warp 4.0 ("Merlin") is a network
   operating system.

   (http://mit.edu:8001/activities/os2/os2world.html).

   [Dates?]

   [Jargon File]

   (1995-07-20)