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OpenMP is a system of so-called "compiler directives" that are used to express parallelism on a shared-memory machine. OpenMP has become an industry standard for such directives, and at this point, most parallel enabled compilers that are used on SMP machines are capable of processing OpenMP directives. The OpenMP standard has had a rather short and steep career: it was introduced in 1997 and has since sidelined all other similar systems.
OpenMP is exclusively designed for shared-memory machines, and is based on "multi-threading", i.e. the dynamic spawning of so-called "light-weight" sub-processes, commonly within loops. In favorable cases it is quite possible to create a well-scaling parallel program from a serial code by inserting a few lines of OpenMP directives into the serial precursor and recompiling. The simplicity and ease of use of OpenMP directives have made it a popular alternative to the more involved (and arguably more powerful) communication system MPI, which was designed for distributed-memory systems.