A polyphonic composition based on one main theme, a cornerstone of baroque music, is called the

A polyphonic composition based on one main theme, a cornerstone of baroque music, is called the fugue.

Explanation:

The fugue is a polyphonic composition (meaning that it's sung in two or more voices) based on one main, principal theme and melodic lines that imitate it. Fugues were the most popular in the baroque period. It is believed that the fugue originated from the canon, a composition in which the parts or voices share the same melody, but do not begin at the same time. Examples of compositors who wrote fugues are Johann Sebastian Bach, Wolfgang Amadeus Mozart, and Ludwig van Beethoven.