“In physical sciences, a nonlinear system is a system in which the output is not directly proportional to the input. Nonlinear problems are of interest to engineers, physicists and mathematicians and many other scientists because most systems are inherently nonlinear in nature. Nonlinear systems may appear chaotic, unpredictable or counterintuitive, contrasting with the much simpler linear systems.”

–Wikipedia

I wondered what the use of the word nonlinear over time was. I found Google Books and they track such things with their Ngram Viewer.

The use of the word nonlinear in books is decidedly nonlinear.