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A duty cycle is a percentage of time over a period when something is active or engaged, in particular, when the activity recurs. An electrical device might be active only 1/10 of each second, for a duty cycle of 10% with a period of 1 second. The term has been used in describing periodic phenomena in science: in astrophysics, the term duty cycle might be used in describing an active galactic nucleus (AGN)'s (somewhat) cyclical activity.