The act of involving. The state of being involved. Intricacy; complexity. Something, such as a long grammatical construction, that is intricate or complex. Mathematics An operation, such as negation, which, when applied to itself, returns the original number. Embryology The ingrowth and curling inward of a group of cells, as in the formation of a gastrula from a blastula. Medicine A decrease in size of an organ, as of the uterus following childbirth. A progressive decline or degeneration of normal physiological functioning occurring as a result of the aging process.