Meaning of nonlinearity

non•lin•e•ar•i•ty

Pronunciation: (non"lin-ē-ar'i-tē), [key]
— n.
  1. the quality of a function that expresses a relationship that is not one of direct proportion.
  2. deviation of an input-output relationship from one of direct proportionality.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.