Thanks for your input, but you are mixing apples with oranges here. A carbon offset is more of a unit of measure. One carbon offset represents the reduction of one metric ton of carbon dioxide or its equivalent in other greenhouse gases... Carbon sequestration is often used in this context, but it is a term describing processes that remove carbon from the atmosphere... I assure you that the term in question is a sink or "carbon/CO2 sink". The main natural sinks are (1) the oceans and (2) plants and other organisms that use photosynthesis to remove carbon from the atmosphere by incorporating it into biomass and release oxygen into the atmosphere...
Long story short, here is a Wikipedia link to support this:
http://en.wikipedia.org/wiki/Carbon_sink