The term is used in different contexts in Bach's time. Most often, it means a purely instrumental work, serving as an introduction to a larger-scale work, such as a cantata, or a part of a cantata (as in Cantata 75 and 76, which are written in two parts). In this context, the term is a synonym for prelude or overture (in the modern sense). Bach also used the term "sinfonia" for this 3-part inventions for keyboard. In that case, these are stand-alone works.