A trigger that marks the situation when and where the common equity tier 1 ratio (CET1) of a bank or any regulated entity drops below a pre-specified trigger level, in which case a CoCo bond (CoCo debt) is automatically written down or converted into equity (of the same entity). For example, the trigger level may be set at a CET1 ratio of 8%.

In other words, the CET1 trigger level is the level of the CET1 ratio below which the CoCo debt (issued by the bank) is written down or converted into shares (i.e. when CET1 ratio < trigger level, which is usually set at 7% for a high trigger and 5.125% for a low trigger).