Source : Free On-Line Dictionary of Computing

Modified Frequency Modulation
     
         (MFM, Modified {FM}, or sometimes "Multiple
        Frequency Modulation") A modification to the original
        {frequency modulation} scheme for encoding data on {magnetic
        disks}.  MFM allows more than 1 symbol per flux transition (up
        to 3), giving greater density of data.  It is used with a data
        rate of between 250-500 kbit/s on industry standard 3.5" and
        5.25" low and high density {diskettes}, and up to 5 Mbit/s on
        {ST-506} {hard disks}.  Except for 1.44 MB floppy disks, this
        encoding is obsolete.
     
        Other data encoding schemes include {GCR}, {FM}, {RLL}.  See
        also: {PRML}.
     
        (2002-06-24)