Definition and background:
A drone. A pipe or string producing a constant pitch. It is analogous to the English " burden ", used to describe the lowest drone on the hurdy-gurdy and the low- pitched, free vibrating strings of the larger lutes such as the theorbo and bowed instruments such as the lira da braccio; also the lowest partial (" hum note ") of tower bells.
For more information on the music term "Bourdon" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary