Carbon reservoirs and conditions that take in and store more carbon (carbon sequestration) than they release. Carbon sinks can serve to partially offset greenhouse gas emissions. Forests and oceans are common carbon sinks. (EPA)

Drawing CO2 out of the air and into biomass is the only known practical way to remove large volumes of a greenhouse gas from the atmosphere. Carbon dioxide emissions from all human activities make up only a small fraction of the carbon that cycles annually among the atmosphere, terrestrial biota, and oceans. but because CO2 emissions from human activities account for the unwanted build-up of atmospheric CO2, reducing these emissions should moderate the rate and reduce the ultimate magnitude of climate change.

Processes that remove more carbon dioxide from the atmosphere than they release.  Both the terrestrial biosphere and oceans can act as carbon sinks.