patch space

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   contain new code, and the superseded code is patched to contain a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 widening use of HLLs has
   made this term rare; it is now primarily historical outside IBM
   shops.  See {patch} (sense 4), {zap} (sense 4), {hook}.