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

Music theory

Music theory is the study of how music works. It examines the language and notation of music. It seeks to identify patterns and structures in composers' techniques across or within genres, styles, or historical periods...

 books in the eighteenth-century, but was used by composers such as Arcangelo Corelli

Arcangelo Corelli

Arcangelo Corelli was an Italian violinist and composer of Baroque music.-Biography:Corelli was born at Fusignano, in the current-day province of Ravenna, although at the time it was in the province of Ferrara. Little is known about his early life...

 before this time. C.P.E. Bach commented that, in practice, Italians did not play tasto solo.