Source : Free On-Line Dictionary of Computing

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        code, and the superseded code is patched to contain a jump or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}]