Polynomial regression involves adding higher-degree terms of the independent variable to the model. For instance, a second-degree polynomial regression would include terms like X and X2. The general form of a polynomial regression model is:
Y = β0 + β1X + β2X2 + ... + βnXn + ε
Here, β0, β1, ..., βn are the coefficients to be determined, and ε is the error term.