Downtime refers to any period when a machine, system, or process is not operational or unavailable for its intended use. 

In manufacturing, it's a critical metric representing lost production time due to breakdowns, maintenance, changeovers, or other interruptions. High downtime directly impacts productivity, increases costs, and can disrupt entire supply chains. Minimizing downtime is a primary goal for factory managers seeking to maximize efficiency, profitability, and overall operational performance.