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]

WordNet (r) 3.0 (2006):

nonlinear
    adj 1: designating or involving an equation whose terms are not
           of the first degree [ant: additive, linear]

The Jargon File (version 4.4.7, 29 Dec 2003):

nonlinear
 adj.

    [scientific computation]

    1. Behaving in an erratic and unpredictable fashion; unstable. When used to
    describe the behavior of a machine or program, it suggests that said
    machine or program is being forced to run far outside of design
    specifications. This behavior may be induced by unreasonable inputs, or may
    be triggered when a more mundane bug sends the computation far off from its
    expected course.

    2. When describing the behavior of a person, suggests a tantrum or a flame
    . ?When you talk to Bob, don't mention the drug problem or he'll go
    nonlinear for hours.? In this context, go nonlinear connotes ?blow up out
    of proportion? (proportion connotes linearity).


The Free On-line Dictionary of Computing (30 December 2018):

nonlinear

   (Scientific computation) A property of a system whose output
   is not proportional to its input.  For example, a transistor
   has a region of input voltages for which its output voltage is
   found by multiplying the input voltage by the gain of the
   transistor.  Outside this region though, the transistor
   behaves non-linearly, meaning that it does not obey this
   simple equation.  The behaviour of a system containing
   non-linear components is thus harder to model and to predict.

   [Jargon File]