Source : Free On-Line Dictionary of Computing

context switch
     
         When a {multitasking} {operating system}
        stops running one {process} and starts running another.  Many
        operating systems implement concurrency by maintaining
        separate environments or "contexts" for each process.  The
        amount of separation between processes, and the amount of
        information in a context, depends on the operating system but
        generally the OS should prevent processes interfering with each
        other, e.g. by modifying each other's memory.
     
        A context switch can be as simple as changing the value of the
        {program counter} and {stack pointer} or it might involve
        resetting the {MMU} to make a different set of memory {pages}
        available.
     
        In order to present the user with an impression of parallism,
        and to allow processes to respond quickly to external events,
        many systems will context switch tens or hundreds of times per
        second.
     
        (1996-12-18)