Removal unit. A Kyoto Protocol unit equal to 1 metric tonne of carbon dioxide (CO2). RMUs can be earned by developed countries (Annex I Parties) through investing in LULUCF activities that absorb or remove CO2 from the atmosphere such as reforestation (see LULUCF). RMUs can be traded and sold under the emission trading mechanism (see Emissions trading), and used by developed countries to meet their emission reduction targets under the Kyoto Protocol.

unfccc.int (general source)
Wikipedia.org (Removal Units)