Source : Free On-Line Dictionary of Computing

Dual Tone Multi Frequency
     
         (DTMF, or "touch-tone") A method used by the
        telephone system to communicate the keys pressed when
        dialling.  Pressing a key on the phone's keypad generates two
        simultaneous tones, one for the row and one for the column.
        These are decoded by the exchange to determine which key was
        pressed.
     
        (1995-03-28)