Counterfactual Outcome
The counterfactual outcome for a unit is the potential outcome corresponding to the treatment level the unit did not receive.
If a treated unit reveals $Y_i(1)$, then $Y_i(0)$ is counterfactual. If a control unit reveals $Y_i(0)$, then $Y_i(1)$ is counterfactual.
Counterfactuals are central because causal effects compare observed outcomes with unobserved alternatives.