The Collaborative International Dictionary of English v.0.48:

Pentachord \Pen"ta*chord\, n. [L. pentachordus five-stringed,
   Gr. penta`chordos; penta- five + chordh` string.]
   1. An ancient instrument of music with five strings.
      [1913 Webster]

   2. An order or system of five sounds. --Busby.
      [1913 Webster]