Source : Free On-Line Dictionary of Computing

explicit parallelism
     
        A feature of a programming language for a {parallel
        processing} system which allows or forces the programmer to
        annotate his program to indicate which parts should be
        executed as independent parallel tasks.  This is obviously
        more work for the programmer than a system with {implicit
        parallelism} (where the system decides automatically which
        parts to run in parallel) but may allow higher performance.