H. O. Hartley, The Modified Gauss-Newton Method for the Fitting of Non-Linear Regression Functions by Least Squares, Technometrics, Vol. 3, No. 2 (May, 1961), pp. 269-280 ...

Components which show I–V graphs that are not straight lines are non-linear circuit elements. The current through them is not directly proportional to the potential difference across them ...