Definition: Comparison of potential outcomes, same unit, same moment in time post-treatment (we only observe 1) (Imbens and Rubin 2015)
Q: What if we change the moment in time? Why does it matter? (see here)
References
Imbens, Guido W, and Donald B Rubin. 2015. Causal Inference for Statistics, Social, and Biomedical Sciences: An Introduction. Cambridge University Press.