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patch: 1. n. A temporary addition to a piece of code, usually as a
   quick-and-dirty remedy to an existing bug or misfeature.  A
   patch may or may not work, and may or may not eventually be
   incorporated permanently into the program.  Distinguished from a
   diff or mod by the fact that a patch is generated by more
   primitive means than the rest of the program; the classical
   examples are instructions modified by using the front panel
   switches, and changes made directly to the binary executable of a
   program originally written in an HLL.  Compare one-line
   fix.  2. vt. To insert a patch into a piece of code.  3. [in the
   UNIX world] n. A diff (sense 2).  4. A set of modifications to
   binaries to be applied by a patching program.  IBM operating
   systems often receive updates to the operating system in the form
   of absolute hexadecimal patches.  If you have modified your OS, you
   have to disassemble these back to the source.  The patches might
   later be corrected by other patches on top of them (patches were
   said to "grow scar tissue").  The result was often a convoluted
   patch space and headaches galore.  5. [UNIX] the
   `patch(1)' program, written by Larry Wall, which automatically
   applies a patch (sense 3) to a set of source code.

There is a classic story of a tiger team penetrating a secure military computer that illustrates the danger inherent in binary patches (or, indeed, any patches that you can't --- or don't --- inspect and examine before installing). They couldn't find any trap doors or any way to penetrate security of IBM's OS, so they made a site visit to an IBM office (remember, these were official military types who were purportedly on official business), swiped some IBM stationery, and created a fake patch. The patch was actually the trapdoor they needed. The patch was distributed at about the right time for an IBM patch, had official stationery and all accompanying documentation, and was dutifully installed. The installation manager very shortly thereafter learned something about proper procedures.