Efficacy
A measure of how effective a radiative forcing from a given anthropogenic or natural mechanism is at changing the equilibrium global surface temperature compared to an equivalent radiative forcing from carbon dioxide. A carbon dioxide increase by definition has an efficacy of 1.0.
Definition courtesy of IPCC AR4.
All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.