Transactions for the sale of permits, reductions or offsets together comprise the carbon market. In fact, Carbon dioxide is only one of the several greenhouse gases that can be "traded". There is still no single, unified international market for emission reduction purchases. Instead, there are various markets, in operation around the world, which can be classified as either "regulatory" or "voluntary" markets and which interact with one another in different ways. See Carbon Trading
"Glossary and Abbreviations", Barnsley, Ingrid, United Nations University Institute of Advanced Studies (UNU-IAS) 2009, "UNUIAS Guide, Reducing Emissions from Deforestation and Degradation in Developing Countries (REDD): A Guide for Indigenous Peoples." January 2009 http://www.ias.unu.edu/resource_centre/2009_REDD_Guide.pdf
A popular (but misleading) term for a trading system through which countries may buy or sell units of greenhouse-gas emissions in an effort to meet their national limits on emissions, either under the Kyoto Protocol or under other agreements, such as that among member states of the European Union. The term comes from the fact that carbon dioxide is the predominant greenhouse gas, and other gases are measured in units called "carbon-dioxide equivalents."