nonlinear

Adjective

non=lin=e=ar

WORDNET DICTIONARY

nonlinear(a = adj.all) - designating or involving an equation whose terms are not of the first degree;

nonlinear, adj.

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]
Containing variables of greater than the first degree; -- of an equation. Opposite of linear. [PJC]
Represented by equations containing variables of greater than the first degree; -- of physical processes or relationships. Opposite of linear. [PJC]