Definition of "nonlinear"

(of a set of points) Not lying on a straight line.  examples

(chemistry, of a molecule) Whose atoms do not lie in a straight line.  examples

(mathematics, of a function) Having a product of independent variables, or a variable with an exponent not equal to one.  examples

(of a system) Whose output is not directly proportional to its input.  examples

Erratic and unpredictable; tending to jump back and forth.  examples