Contingent convertible capital
From ACT Wiki
Contingent convertible capital is made up of hybrid capital securities that, through a conversion mechanism, provide additional capital available to absorb losses when the capital of the issuing institution falls below a certain level. They are generally used by banks in meeting regulatory capital requirements.
Depending on its terms, contingent convertible capital may be treated by regulators either as Additional Tier 1 (AT1) capital, or as Tier 2 (T2) capital.
The BIS's quarterly report of September 2013 has a useful primer on CoCos.