Is a "curve" considered "linear"?
Polynomial regression (for nth degree polynomial) in statistics is a special case of linear regression. Lets give an example for square function:

1. y = w*x           



  This is linear in terms of both weight (w) and data (x).  


2. y = w*(x^2)    OR        y = w*z ; where z = x^2     



  This is still linear in terms of weight (w) and still treated as a linear regression for the transformed data (z). While the modeled relationship between y and x is certainly non-linear.


As you can notice above: The commonality in (1) and (2) is the linearity with the weight/ coefficient of linear regression.