Carbon-12

Carbon-12 (also written 12C) is an isotope of carbon. It is the most common isotope of carbon, as 98.89% of all carbon is this type of carbon. It is one of two stable (non-radioactive) isotopes of carbon. The other is carbon-13.
Carbon-12 is important in chemistry as it is the benchmark from which all other relative atomic masses are worked out. Its relative atomic mass is exactly 12.