In perpetuity Definition 1 of 2Pro Tip 1/3Eternal Commitment Use this phrase to express an unbreakable, ongoing promise or agreement. The artwork was donated to the museum in perpetuity. Pro Tip 2/3No End Date 'In perpetuity' indicates an arrangement without a specific time limit. Her name will be displayed in perpetuity as a benefactor of the institution. Pro Tip 3/3Legal Duration Frequently used in legal contexts to imply duration that lasts as long as the law exists. The rights to the land were granted to them in perpetuity.