Polynomial Regression

Linear or not?

https://en.wikipedia.org/wiki/Linear_regression#/media/File:Polyreg_scheffe.svg

Example of a cubic polynomial regression, which is a type of linear regression. 
Although polynomial regression fits a nonlinear model to the data, 
as a statistical estimation problem it is linear, 
in the sense that the regression function E(y | x) is linear in the unknown parameters 
that are estimated from the data. 
For this reason, polynomial regression is considered to be a special case of multiple linear regression.

[[https://en.wikipedia.org/wiki/Linear_regression | Wikipedia: Linear Regression]]