The general meaning of a carbon leakage corresponds to "the ratio of carbon emissions increase from a specific sector outside the country (as a result of a policy affecting that sector in the country) over the carbon emission reductions in the sector (again, as a result of the environmental policy)".
(IEA, Issues behind Competitiveness and Carbon Leakage, Focus on Heavy Industry, IEA Information paper, October 2008)
The definition of "carbon leakage" in the UNFCCC-related literature can be "strong" or "weak".
In the strong definition, the carbon leakage is equivalent to the increase of CO2 emissions in non-Annex B countries divided by the reduction of CO2 emissions in Annex B countries.
(IPCC, Climate change 2007 : Mitigation, Contribution of WGIII to the Fourth Assessment Report, 2007)
However, this definition ignores the fact that non-Annex B emissions may raise for other reasons than only the effects of the climate mitigation mechanisms in Annex B countries (cf. flexible mechanisms, shift of production due to too strict environmental legislation in Annex B countries, etc.).
(Peters G.P. and Hertwich E.G., CO2 embodied in international trade with implications for global climate policy, Environment Science and Technology, Accepted December 18, 2007)
In the weak definition, the carbon leakage corresponds to the CO2 emissions embodied in imports from non-Annex B countries to Annex B countries.
(Peters G.P. and Hertwich E.G., CO2 embodied in international trade with implications for global climate policy, Environment Science and Technology, Accepted December 18, 2007)
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