# Is an interaction term considered as a covariate?

If I have a model y ~ A*B which is y ~ A+B+AB, is the interaction term AB considered as a covariate or not? I'm not sure whether I should say that there are three covariates or two covariates in this model. Thank you.

The terminology gets even worse. Let’s say that A is a binary variable denoting group membership, control vs treatment, and B is some other source of variability. If you’re interested in if the treatment has any impact and run ANCOVA with $$\hat{y}=\beta_0+\beta_1A +\beta_2B$$, I would call A the group variable and B a covariate. If you throw in the interaction term, then I would say there is a group variable, a covariate, and their interaction.