
'single key'; used on a continuo part to indicate that the notes should be played without harmony
Found on
http://en.wikipedia.org/wiki/Glossary_of_musical_terminology

Tasto solo is an Italian term used in music scores, usually on the continuo part, to indicate that a note or section should be played on its own, without harmony. The phrase first appeared in music theory books in the eighteenth-century, but was used by composers such as Arcangelo Corelli before this time. C.P.E. Bach commented that, in practice, ...
Found on
http://en.wikipedia.org/wiki/Tasto_solo

In old music, a tasto solo was 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.
Found on
http://www.probertencyclopaedia.com/browse/VT.HTM
No exact match found.