Webster’s Revised Unabridged Dictionary · 1913
Tasto
Tas′to (täs′tō), 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.