Senior Security
A security (usually debt) that, in the event the issuer goes bankrupt, must be repaid before other creditors receive any payment.
Investopedia Commentary
In the event of liquidation, senior debt holders have seniority and are repaid before the junior debt.
This is also known as "unsubordinated debt."
Related Links
Bond Basics Tutorial
See also: Absolute Priority, Bankruptcy, Bond, Junior Security, Liquidation, Subordinated Debt, Unsubordinated Debt