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

implicit parallelism

    A feature of a programming language for a parallel
   processing system which decides automatically which parts to
   run in parallel.

   The best way of providing implicit parallelism is still (1995)
   an active research topic.  The problem is to generate the
   right number of parallel tasks of the right size (or
   "granularity").  Too many tasks and the system gets bogged
   down in house-keeping, or memory for waiting tasks runs out,
   too few tasks and processors are left idle.

   The best performance is usually achieved with explicit
   parallelism where the programmer can annotate his program to
   indicate which parts should be executed as independent
   parallel tasks.

   (1995-02-16)