nonlinear

Short definition from Wiktionary

Adjective

1. 1
   Not lying on a straight line.
2. 2
   Whose atoms do not lie in a straight line.
3. 3
   Having a product of independent variables, or a variable with an exponent not equal to one.
4. 4
   Whose output is not directly proportional to its input.
5. 5
   Erratic and unpredictable; tending to jump back and forth.