From The Collaborative International Dictionary of English v.0.48 :
nonlinear \nonlinear\ adj.
1. (Math.) Not depictable graphically as a straight line; not
changing by a constant amount for each unit of time,
distance, or other independent variable. Opposite of
linear.
[WordNet 1.5 +PJC]
2. (Math.) Containing variables of greater than the first
degree; -- of an equation. Opposite of linear.
[PJC]
3. (Physics) Represented by equations containing variables of
greater than the first degree; -- of physical processes or
relationships. Opposite of linear.
[PJC]