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

upper memory block

    (UMB) Up to 64 kilobytes of the expanded memory
   page frame above the first 64 kilobytes.  The UMB can be used
   to store TSR programs or device drivers thereby freeing
   parts of the precious conventional memory.  The UMB is
   provided by special memory manager programs; many EMMs can
   provide UMB as well.

   (1996-01-10)