Is it ensure, insure, or assure?
involution in·vo·lu·tion (ĭn'və-lōō'shən)
A decrease in size of an organ, as of the uterus following childbirth.
The ingrowth and curling inward of a group of cells, as in the formation of a gastrula from a blastula.
A progressive decline or degeneration of normal physiological functioning occurring as a result of the aging process. Also called catagenesis.