| Noun | 1. | downtime - a period of time when something (as a machine or factory) is not operating (especially as a result of malfunctions) period, period of time, time period - an amount of time; "a time period of 30 years"; "hastened the period of time of his recovery"; "Picasso's blue period" uptime - a period of time when something (as a machine or factory) is functioning and available for use |