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the superseded code is patched to contain a jump or call to the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.