Carbon sequestration is the process of storing carbon in a carbon pool. It plays a crucial role in limiting climate change by reducing the amount of carbon dioxide in the atmosphere. There are two main types of carbon sequestration: biologic (also called biosequestration) and geologic.

Biologic carbon sequestration is a naturally occurring process as part of the carbon cycle. Humans can enhance it through deliberate actions and use of technology. Carbon dioxide (CO
₂) is naturally captured from the atmosphere through biological, chemical, and physical processes. These processes can be accelerated for example through changes in land use and agricultural practices, called carbon farming. Artificial processes have also been devised to produce similar effects. This approach is called carbon capture and storage. It involves using technology to capture and sequester (store) CO
₂ that is produced from human activities underground or under the sea bed.

Plants, such as forests and kelp beds, absorb carbon dioxide from the air as they grow, and bind it into biomass. However, these biological stores may be temporary carbon sinks, as long-term sequestration cannot be guaranteed. Wildfires, disease, economic pressures, and changing political priorities may release the sequestered carbon back into the atmosphere.

Carbon dioxide that has been removed from the atmosphere can also be stored in the Earth's crust by injecting it underground, or in the form of insoluble carbonate salts. The latter process is called mineral sequestration. These methods are considered non-volatile because they not only remove carbon dioxide from the atmosphere but also sequester it indefinitely. This means the carbon is "locked away" for thousands to millions of years.

To enhance carbon sequestration processes in oceans the following chemical or physical technologies have been proposed: ocean fertilization, artificial upwelling, basalt storage, mineralization and deep-sea sediments, and adding bases to neutralize acids. However, none have achieved large scale application so far. Large-scale seaweed farming on the other hand is a biological process and could sequester significant amounts of carbon. The potential growth of seaweed for carbon farming would see the harvested seaweed transported to the deep ocean for long-term burial. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" on seaweed farming as a mitigation tactic.

The term carbon sequestration has diverse meanings in the literature and media. The IPCC Sixth Assessment Report defines carbon sequestration as "The process of storing carbon in a carbon pool" . Subsequently, a pool is defined as "a reservoir in the Earth system where elements, such as carbon and nitrogen, reside in various chemical forms for a period of time".

The United States Geological Survey (USGS) defines carbon sequestration as follows: "Carbon sequestration is the process of capturing and storing atmospheric carbon dioxide." Because the wording in this definition makes it very similar to the definition of carbon capture and storage (CCS), carbon sequestration is sometimes confounded with CCS (the IPCC defines CCS as "a process in which a relatively pure stream of carbon dioxide (CO₂) from industrial sources is separated, treated and transported to a long-term storage location" ).

Carbon sequestration is part of the natural carbon cycle by which carbon is exchanged among the biosphere, pedosphere (soil), geosphere, hydrosphere, and atmosphere of Earth. Carbon dioxide is naturally captured from the atmosphere through biological, chemical, or physical processes, and stored in long-term reservoirs.