WordNet (r) 3.0 (2006):

subroutine
    n 1: a set sequence of steps, part of larger computer program
         [syn: routine, subroutine, subprogram, procedure,
         function]

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

subroutine
procedure
routine

    (Or "procedure") A sequence of instructions
   for performing a particular task.  Most programming languages,
   including most machine languages, allow the programmer to
   define subroutines.  This allows the subroutine code to be
   called from multiple places, even from within itself (in which
   case it is called recursive).  The programming language
   implementation takes care of returning control to (just after)
   the calling location, usually with the support of call and
   return instructions at machine language level.

   Most languages also allow arguments to be passed to the
   subroutine, and one, or occasionally more, return values to
   be passed back.

   A function is often very similar to a subroutine, the main
   difference being that it is called chiefly for its return
   value, rather than for any side effects.

   (1996-10-01)