A nuisance function supports estimation of a primary causal parameter without being the main quantity of interest. In causal inference, the propensity score is one example: it helps apply the adjustment formula, while the causal effect remains the target. section 8.1.3
Propensity score in adjustment
Suppose an analysis estimates a causal effect using the adjustment formula. The propensity score describes a function needed for that estimation, but attention remains on the causal parameter rather than on the propensity score itself. section 8.1.3
