Source : Free On-Line Dictionary of Computing

main memory
     
         The storage device used by a computer
        to hold the currently executing program and its working data.
        A modern computer's main memory is built from {random access
        memory} {integrated circuits}.  In the old days {ferrite core
        memory} was one popular form of main memory, leading to the
        use of the term "{core}" for main memory.
     
        Computers have several other sorts of memory, distinguished by
        their access time, storage capicity, cost, and the typical
        lifetime or rate of change of the data they hold.  {Registers}
        in the {CPU} are fast, few, expensive and typically change
        every few machine instructions.  Other kinds are {cache},
        {PROM}, {magnetic disk} (which may be used for {virtual
        memory}), and {magnetic tape}.
     
        (1996-11-04)