Definition:
Every manufacturing process has periods of time where equipment is unavailable due to tooling changes, material changes, part changes, program changes, or any other changes to production that must be performed while equipment is stopped. Collectively, these events are referred to as “changeovers”.
In order to measure changeover time accurately, it is important to create a clearly defined standard and then consistently apply that standard
Further Reading:
Book: The Lean Startup by Eric Ries