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

dead code
 n.

    Routines that can never be accessed because all calls to them have been
    removed, or code that cannot be reached because it is guarded by a control
    structure that provably must always transfer control somewhere else. The
    presence of dead code may reveal either logical errors due to alterations
    in the program or significant changes in the assumptions and environment of
    the program (see also software rot); a good compiler should report dead
    code so a maintainer can think about what it means. (Sometimes it simply
    means that an extremely defensive programmer has inserted can't happen
    tests which really can't happen ? yet.) Syn. grunge. See also dead, and
    The Story of Mel'.


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

dead code
infeasible path

    (Or "infeasible path") Any part of a program
   that can never be executed because no control flow path
   leads to it.  This may be because it is guarded by a control
   structure that will always transfer control somewhere else,
   e.g.

     if (false)
     
       # dead code
     

   or it may be for a less obvious, less local reason, e.g. code
   in a function that is only called from another function that
   is only called to handle certain input that never occurs in
   practice.

   Determining that some code is dead may thus require analysis
   of the whole program.  Consideration of possible inputs is
   probably beyond the normal (static) identification of dead
   code.

   The presence of dead code may reveal either logical errors due
   to alterations in the program or significant changes in the
   assumptions and environment of the program (see also software
   rot).  Sometimes it simply represents can't happen tests
   inserted by a defensive programmer.

   A good compiler should warn about dead code or it may
   perform dead code elimination.

   [Jargon File]

   (2018-08-19)