Source : WordNet®

sequencer
     n 1: (chemistry) an apparatus that can determine the sequence of
          monomers in a polymer [syn: {sequenator}]
     2: computer hardware that sorts data or programs into a
        predetermined sequence

Source : Free On-Line Dictionary of Computing

sequencer
     
         Any system for recording and/or playback of music via
        a programmable memory which stores music not as audio data,
        but as some representation of notes.  The most common modern
        usage of "sequencer" is to refer to systems (whether in
        software, or as a feature of devices like synthesizers or drum
        machines) that deal with {MIDI} data.
     
        (1999-06-04)