it actually encompasses any unplanned event that causes your manufacturing process to stop. For example down time can be triggered by material issues, a shortage of operators, or unscheduled maintenance. The unifying element is that although production is scheduled, the process is not running due to an unplanned stop.
By way of contrast, a planned stop is any event where the process is unavailable to run due to a preplanned activity, such as a changeover or scheduled maintenance.