A Collegium Musicum was a music society.
With the burgeoning of bourgeois culture, particularly in prosperous and vibrant centres such as Leipzig, there began a slow shift away from music being made exclusively for The Court or The Church. Art music was becoming accessible to a much wider public. An example of this is the Music Hall in Dublin where Messiah received its first performance. Collegia musica were a vital part of this development.
Collegia musica brought musicians together to make music, and were often purely amateur in status. In Hamburg, Georg Philipp Telemann set up a collegium musicum in his own home. It was Telemann who founded the Collegium Musicum in Leipzig, in 1702 (when he was also associated with St. Thomas’s for a time). This became the focal point for professional musicians, and it was an organisation of real standing within the city, informal and voluntary institution though it was. Bach’s becoming director of the Collegium does not seem to have brought him any financial rewards, but it did greatly widen his scope, compared to the restrictions placed on him by his ‘day-job’ at the Thomasschule.