The Collaborative International Dictionary of English v.0.48:

Counter tenor \Coun"ter ten`or\ (t?n`?r). [OF. contreteneur. Cf.
   Contratenor, and see Tenor a part in music.] (Mus.)
   One of the middle parts in music, between the tenor and the
   treble; high tenor.
   [1913 Webster]

   Counter-tenor clef (Mus.), the C clef when placed on the
      third line; -- also called alto clef.
      [1913 Webster]