Carbon and credits
Carbon credit
A carbon credit is a tradable certificate representing one tonne of carbon dioxide, or its equivalent in another greenhouse gas, either kept out of the atmosphere or taken back out of it. It is created by a project, verified by a third party, and destroyed when someone claims it.
The unit is always one tonne. What differs between credits, and what decides what they are worth, is how that tonne was accounted for: whether the project would have happened anyway, how long the carbon stays put, and who checked.
A credit is only meaningful once it is retired, which means permanently removed from circulation so nobody else can claim the same tonne. An unretired credit sitting in a registry account has done nothing.