A sinfonietta is a symphony that is smaller in scale (either in terms of length or the instrumental forces required), or lighter in approach than a standard symphony.
It appears to have been coined in 1874 by Joachim Raff for his Op.
188, but became common usage only in the early 20th century (Temperley 2001).
Just as the term symphony itself can refer to pieces of music of varied size and scope, it is difficult to identify common criteria which pieces called sinfonietta share.
Many of the sinfoniettas listed on this page employ larger forces and/or are longer than pieces designated symphonies, sometimes even by the same composer.