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

patch space
 n.

    An unused block of bits left in a binary so that it can later be modified
    by insertion of machine-language instructions there (typically, the patch
    space is modified to contain new code, and the superseded code is patched
    to contain a jump or call to the patch space). The near-universal use of
    compilers and interpreters has made this term rare; it is now primarily
    historical outside IBM shops. See patch (sense 4), zap (sense 4), hook
    .


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

patch space

   An unused block of bits left in a binary so that it can later
   be modified by insertion of machine-language instructions
   there (typically, the patch space is modified to contain new
   code, and the superseded code is patched to contain a jump or
   call to the patch space).  The widening use of HLLs has made
   this term rare; it is now primarily historical outside IBM
   shops.  See patch, zap, hook.

   [Jargon File]