As a verb, solo is a musical term that most generally means to play or sing alone. This does not require that there be absolutely no other players or singers, only that the soloist’s part is more prominent than any accompaniment. Composers of orchestral music often specify that certain parts be played “solo”, (for example, only one of the four French horns plays), to change the texture and timbre of the piece.
As a noun, solo refers to a piece of music played predominantly by one performer, often with other performers providing less prominent accompaniment. This is how the term is most commonly understood today. Solos often require a musician to display his or her virtuosity, often calling upon speed and technique. The development of Jazz through the 1900s was in large part based upon players’ abilities to improvise solos on top of the chord progressions of blues and popular songs.