Sinfonia

2)  A name chosen by Bach for his three-part inventions

3)  In the Baroque period (1600-1750), name for orchestral pieces of Italian origin, designed to serve as an introduction to:

a)  an opera or operatic scene (Monteverdi, "Orfeo," 1607;  "L'Incoronazione di Popea,"  1642)
b)  an orchestral suite (S. Rossi, "Sinfonie e gagliarde," 1607)
c)  a cantata (Bach, "Christ lag in Todesbanden")

No fixed form or style attaches to these pieces.  In this period, "sinfonia" is simply one of various names used for an introductory instrumental piece.  Possibly, the term had the connotation of orchestral performance, not necessarily implied by the other terms.

Not until c. 1690 did the operatic sinfonia become standardized (by A. Scarlatti) into what is usually called "Italian Overture," one of the ancestors of the modern symphony.