The Wikipedia article on Nonlinearity states: (emphasis theirs) Ref: http://en.wikipedia.org/wiki/Nonlinearity Interestingly, the page regarding nonlinearity (disambiguation) states: (Not sure what "disproportionate" truly means here either.) Although the Nonlinearity article specifically says that it is describing the use of the term in mathematics, the article clearly implies the use of the term to physical systems, as does the disambiguation article. Examples given at the end of the page include the Navier-Stokes equations for fluid flow. What's your take on this? Is a physical system, such as the flow of a fluid as described by the Navier-Stokes equations, a system "whose behavior is not expressible as a sum of the behaviors of its descriptors"? By making this statement, is the intent only that one can not describe the flow of fluid mathematically, or is there something else about the flow of fluid which is inherently (strongly) emergent in the sense that it is irreducible to what happens within each volume, element or portion of the fluid field? Also, how exactly do you define "descriptors" in this example? I would assume what is meant would be the fluid's properties such as velocity or momentum for example. What would be a complete list of "descriptors" for a fluid system as described by the NS equations?