Source : Webster's Revised Unabridged Dictionary (1913)

Tasto \Tas"to\, n. [It.] (Mus.)
   A key or thing touched to produce a tone.

   {Tasto solo}, single touch; -- in old music, a direction
      denoting that the notes in the bass over or under which it
      is written should be performed alone, or with no other
      chords than unisons and octaves.