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space is modified to contain new code, and the superseded code is patched to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.

