Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {Counter-tenor clef} (Mus.), the C clef when placed on the
      third line; -- also called {alto clef}.