sinfonia
A three-voice contrapuntal composition for keyboard, or in earlier usage, an instrumental introduction to an opera or cantata
In Depth
The term sinfonia has multiple meanings. Bach's 15 Sinfonias (BWV 787–801) are three-part inventions — contrapuntal studies that extend the technique of the two-part inventions into three voices. In Baroque opera, sinfonia referred to the orchestral overture, particularly the Italian overture form (fast-slow-fast) that eventually evolved into the Classical symphony. The dual meaning reflects the word's original sense: a sounding together of multiple parts.
Bach's Sinfonias are often still called three-part inventions, a practical nickname that has stuck despite Bach's own preference for the term Sinfonia.