What is a musical piece in which one solo instrument (such as a piano or violin) is accompanied by an orchestra?

A musical piece in which one solo instrument (such as a piano or violin) is accompanied by an orchestra is called a concerto. Concerto is basically an Italian word for concert, and refers to a large scale musical composition that includes a large concert band that accompanies a solo instrument that is the most important element of that concerto. The band is used as back music.