The Jargon File (version 4.4.7, 29 Dec 2003):

live data
 n.

    1. Data that is written to be interpreted and takes over program flow when
    triggered by some un-obvious operation, such as viewing it. One use of such
    hacks is to break security. For example, some smart terminals have commands
    that allow one to download strings to program keys; this can be used to
    write live data that, when listed to the terminal, infects it with a
    security-breaking virus that is triggered the next time a hapless user
    strikes that key. For another, there are some well-known bugs in vi that
    allow certain texts to send arbitrary commands back to the machine when
    they are simply viewed.

    2. In C code, data that includes pointers to function hooks (executable
    code).

    3. An object, such as a trampoline, that is constructed on the fly by a
    program and intended to be executed as code.


The Free On-line Dictionary of Computing (30 December 2018):

live data

   1. Data that is written to be interpreted and takes over
   program flow when triggered by some un-obvious operation, such
   as viewing it.  One use of such hacks is to break security.
   For example, some smart terminals have commands that allow one
   to download strings to program keys; this can be used to write
   live data that, when listed to the terminal, infects it with a
   security-breaking virus that is triggered the next time a
   hapless user strikes that key.  For another, there are some
   well-known bugs in vi that allow certain texts to send
   arbitrary commands back to the machine when they are simply
   viewed.

   2. In C, data that includes pointers to functions
   (executable code).

   3. An object, such as a trampoline, that is constructed on
   the fly by a program and intended to be executed as code.

   4. Actual real-world data, as opposed to "test data".  For
   example, "I think I have the record deletion module finished."
   "Have you tried it out on live data?"  This usage usually
   carries the connotation that live data is more fragile and
   must not be corrupted, or bad things will happen.  So a more
   appropriate response to the above claim might be: "Well, make
   sure it works perfectly before we throw live data at it."  The
   implication here is that record deletion is something pretty
   significant, and a haywire record-deletion module running amok
   on live data would probably cause great harm.

   [Jargon File]