Linear is a property of straight lines, of simple proportions, of predictability and good behaviour. Nonlinear on the other hand applies to systems that do unpredictable things, that cannot be solved exactly and need to be approximated, the one-offs that don't fit the expected pattern. It comes as a surprise to many people that the natural state of affairs in the world is non-linear.
Linearity is merely an approximation.
Let us demonstrate this by a simple thought experiment. One of the most frequent demonstrations of Newtonian linear dynamics is the billiard ball model of atoms. Conservation of momentum and energy allow us to predict exactly the trajectories of atoms moving in straight lines. Unfortunately the answer is a qualified no. Real Life Predictability Let us take the air itself.