Source : Free On-Line Dictionary of Computing

Matrix Math eXtensions
     
         (MMX) (NOT an acronym for "MultiMedia eXtension",
        according to Intel, but an Intel brand name) A set of 57 extra
        instructions built into some versions of {Intel}'s {Pentium}
        {microprocessors} for supporting {SIMD} operations on
        {multimedia} and communications data types.
     
        MMX-enhanced processors are due to be released early in 1997.
        They will be fully compatible with previous Intel processors
        and software but software will only benefit if it is written
        to use the new instructions.  They can handle many common
        multimedia operations, such as {digital signal processing},
        normally handled by a separate {sound card} or {video card}.
     
        (1996-12-21)