Part of Drum-Buffer-Rope Scheduling. A name given to the rate of production of a physical constraint. The beat of the drum dictates the rate that material should flow into the system.
(http://www.tocpractitioner.com/tocglossary.asp)
Other Definitions
Drum – The Drum refers to the CCR that is used to build the schedule around in an operation. (http://www.toc.co.uk/glossary.htm)