The Italian word "solo" means the only one. Music has long been taken to signify so the piece for one voice or instrument, sometimes with the accompaniment, sometimes without it. Perhaps the most famous of works for solo violin is the Bach Chaconne.
J.S. Bach. Chaconne (fragment from Partita No. 2 in d-moll, for solo violin)
Used this word when in some great work there is a separate party for one of the instruments. For example, in Ravel's Bolero cry continuously sounds solo snare drum, clear rhythm beating.
M. Ravel. "Bolero"
performed by the London Symphony orchestra
There are works in which there is a small solo episodes. Playing all the big orchestra. Sound of different instruments, interwoven melodies. And then from the overall sound of the orchestra is allocated one tool with its melody. It began a solo episode. So, symphonic fantasy "Francesca da Rimini Tchaikovsky allocated in the center of the episode is the story of Francesca. Pensive and sad melody played by the clarinet solo, sounds like a sad story of a young woman.
P. I. Tchaikovsky. The story of Francesca from symphonic fantasy "Francesca da Rimini"
Solo called party in the works for any instrument (or voice) with orchestra - piano or violin Concerto, respectively, solo piano or violin party. Their performers are referred to as soloists.