sinfonia
formsin-FOH-nee-ah
A three-voice contrapuntal composition for keyboard, or in earlier usage, an instrumental introduction to an opera or cantata
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.