Evidence-appraisal glossary
Counterfactual
A counterfactual is the outcome that would have happened for the same person or group under a different exposure than the one they actually received. Causal effects are defined as the contrast between an observed outcome and its unobservable counterfactual.
Also called: potential outcome.
Because no individual can be both treated and untreated at the same moment, the counterfactual is never directly seen, a gap known as the fundamental problem of causal inference. Studies estimate it using comparison groups assumed to stand in for what would have happened, and randomization is one way to make that assumption credible. The term names the target of a causal claim, not a quantity you can read straight off the data.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.