What is linear in parameters?
The model of linear regression is linear in parameters.
What does this actually mean?
A model is linear when each term is either a constant or the product of a parameter and a predictor. A linear equation is constructed by adding the results for each term. This constrains the equation to just one basic form: Response=constant+parameter∗predictor+...+parameter∗predictor "Linear in parameters" in Linear Regression, means no parameter appears as an exponent, nor multiplied or divided by another parameter.