A CDO tranche that constitutes the most senior tranche of a CDO (collateralized debt obligation) issue. Given its seniority as to other tranches in the structure, this tranche is typically rated A to AAA, so that in the case of default or the CDO’s collateral otherwise underperforms, scheduled payments to senior tranches take precedence over lower seniority tranches such as mezzanine tranches.

A senior CDO tranche can be perceived as a relatively safe underlying cash bond, with a very low likelihood of default amongst the entire set of tranches in the CDO issue.