The word perpetuity means the property of being perpetual, or lasting forever. The perpetuity of an eternal flame means that it will burn forever, while an ordinary candle flame will eventually extinguish.
First appearing in the 15th century, the noun perpetuity derives from the Latin word perpetuus meaning "continuing throughout." It can mean the quality of being perpetual, continuing forever, or everlasting. If a person sent into exile from their native country is never allowed to return, they have been banished "in perpetuity."