If we have two features, a and b, we can suspect that there's a polynomial relation, such as a2 + ab + b2. We can consider each term in the sum to be a feature—in this example, we have three features. The product ab in the middle is called an interaction. An interaction doesn't have to be a product—although this is the most common choice—it can also be a sum, a difference, or a ratio. If we're using a ratio to avoid dividing by zero, we should add a small constant to the divisor and dividend.

The number of features and the order of the polynomial for a polynomial relation aren't limited. However, if we follow Occam's razor, we should avoid higher-order polynomials and interactions of many features. In practice, complex polynomial relations tend to be more difficult to compute and tend to overfit, but if you really need better results, they may be worth considering.