Additionality
Also called: additional impact, counterfactual test
Additionality asks whether an outcome would have happened anyway without your money. It's the hardest and most important question in both carbon markets and impact investing, because it requires proving a counterfactual. Failing it is what turned a large share of forestry offsets into paper reductions, and it's the reason primary capital into new projects is treated as more credible than secondary market purchases.