involution

Concise Oxford English Dictionary © 2008 Oxford University Press:
involution/ɪnvəˈl(j)uːʃ(ə)n/

▶noun

• 1 Physiology the shrinkage of an organ in old age or when inactive.

• 2 Mathematics a function, transformation, or operator that is equal to its inverse.

• 3 formal the process or state of complication.

– derivatives
involutional adjective.

– origin ME (in the sense ‘(part) curling inwards’): from L. involutio(n-), from involvere (see involve).